TL;DR. A stock option is a contract that gives the buyer the right, but not the obligation, to buy or sell 100 shares of a stock at a fixed price by a fixed date. A call is the right to buy; a put is the right to sell. You pay a premium up front. Once you understand four numbers — underlying, strike, expiration, premium — you can read any option chain. Everything else (the Greeks, strategies, vol surfaces) is just layers on top of those four.
What an option contract actually is
The U.S. Securities and Exchange Commission defines an option as a contract “giving the purchaser the right – but not the obligation – to buy or sell a security at a fixed price within a specific period of time” (SEC investor.gov — Options). That single sentence carries the whole product. The buyer is paying for optionality — the choice to act later if the price has moved their way, and to walk away if it hasn’t.
Listed equity options in the United States have been standardized since the Chicago Board Options Exchange opened on April 26, 1973 with 16 listed call series across a handful of names. Today the Options Clearing Corporation (OCC) sits in the middle of every trade as the central counterparty, guaranteeing that if you exercise, someone delivers (OCC — reference). That standardization is why you can buy an AAPL July $200 call from a stranger on an exchange and trust that 100 shares will actually show up if you exercise.
Calls and puts — the only two atoms
Every options position in the world is built from these two:
- Call option. The right to buy 100 shares at the strike price, on or before the expiration date. Call buyers want the stock to go up.
- Put option. The right to sell 100 shares at the strike price, on or before the expiration date. Put buyers want the stock to go down (or want insurance against a fall).
For every buyer there is a seller (a “writer”) on the other side, who collects the premium and accepts the obligation to deliver if assigned. FINRA puts it plainly: “For the purchaser of an option, the premium paid is your maximum loss,” but sellers of uncovered calls can face theoretically unlimited losses (FINRA — Options). That asymmetry is the entire risk story.
The four numbers on every option chain
Pull up any options chain and you’ll see the same four fields. They’re all you need to read the contract.
- Underlying. The stock or index the option is written on (AAPL, SPX, etc.).
- Strike price. The fixed price at which the option can be exercised. A “$200 strike call” lets the holder buy at $200.
- Expiration date. The date the option ceases to exist. Standard monthly options expire on the third Friday of the month; weekly options expire on most Fridays; some indices (SPX, NDX) have multiple expirations per week.
- Premium. The price you pay (or receive) for the contract, quoted per share. Because a U.S. equity contract represents 100 shares, a quoted premium of $3.00 means $300 of cash per contract (equity option contract size — reference).
A worked example with real numbers
Suppose XYZ stock trades at $100. You buy one 30-day $100 call for $3.00. You’ve paid $300 (3.00 × 100 shares) and you control 100 shares of upside until expiration. Three things can happen:
- Stock rallies to $110. The call is worth at least $10 of intrinsic value ($1,000 per contract). After fees you keep roughly $700 of profit on a $300 outlay — a 230% return on the option versus a 10% return on the stock.
- Stock sits at $100. The call expires worthless. You lose the full $300 premium — capped, and known up front.
- Stock falls to $90. Same outcome — the call expires worthless, you lose $300. Unlike owning the stock, you do not lose more if the stock drops further. That bounded downside is the whole reason buyers pay a premium.
The mirror trade is a $100 put for $3.00. The put buyer profits if XYZ drops below roughly $97 by expiration (strike minus premium), and is capped at $300 of loss if XYZ rises. That symmetry is why puts are the standard hedge for a long stock portfolio — they’re a price floor with a known cost.
Intrinsic value vs. time value
Every option premium has two components: what the option would be worth if it expired right now (intrinsic value) and everything else (time value, also called extrinsic value).
Intrinsic value is simple arithmetic. For a call it is the underlying price minus the strike, floored at zero. For a put it is the strike minus the underlying price, floored at zero (intrinsic value definition — reference). A $100 call with the stock at $105 has $5 of intrinsic value, no matter what time is left.
Time value is whatever else the market is paying. It reflects the probability that the option becomes more valuable before expiration, given the stock’s volatility, the time remaining, and prevailing interest rates. Time value is largest for at-the-money options and decays toward zero as expiration approaches — the bleed traders call “theta”.
Moneyness in one table
| Contract | Stock | Strike | Premium | Intrinsic | Time value | Moneyness |
|---|---|---|---|---|---|---|
| $90 call | $100 | $90 | $11.20 | $10.00 | $1.20 | Deep ITM call |
| $100 call | $100 | $100 | $3.00 | $0.00 | $3.00 | ATM call |
| $110 call | $100 | $110 | $0.60 | $0.00 | $0.60 | OTM call |
| $110 put | $100 | $110 | $11.10 | $10.00 | $1.10 | Deep ITM put |
| $100 put | $100 | $100 | $2.70 | $0.00 | $2.70 | ATM put |
| $90 put | $100 | $90 | $0.50 | $0.00 | $0.50 | OTM put |
Three patterns to notice. First, deep in-the-money options are almost all intrinsic value — they behave like the stock itself. Second, out-of-the-money options are pure time value — their entire premium is the market’s probability bet that the stock will move enough to matter. Third, at-the-money options carry the most time value in absolute terms, because the strike sits exactly at the coin flip.
Payoff diagrams — what you actually own at expiration
The cleanest way to picture an option is its payoff at expiration. Time value has decayed to zero, so the only thing that matters is the intrinsic-value formula. The chart below shows the four basic positions side by side.
How premium decomposes over time
An option’s total premium is intrinsic plus time value at every moment. The chart below traces the two pieces of a 60-day at-the-money call as the calendar advances. Intrinsic value is flat at zero as long as the stock sits at the strike; time value falls along the familiar non-linear “theta curve” that accelerates into the final two weeks.
American vs European — the exercise rule that catches people out
Two exercise styles dominate the listed market. American-style options can be exercised any trading day up to and including expiration; the vast majority of single-stock and ETF options (AAPL, SPY, QQQ) are American. European-style options can only be exercised on the expiration date itself; this includes most cash-settled index options such as SPX and NDX (Cboe SPX contract specs).
The practical takeaway: with American single-stock options, an in-the-money long position can be exercised at any time, and a short position can be assigned at any time — especially around a dividend date. European index options can’t be assigned early, which is why portfolio hedges built on SPX often behave more predictably than the same hedge built on SPY.
Common mistakes that cost beginners money
- Forgetting the 100× multiplier. A $2.50 premium looks small until you remember it’s $250 per contract. Five contracts is $1,250 at risk, not $12.50.
- Buying far out-of-the-money calls because they’re “cheap”. A 0.05-delta call is a 1-in-20 lottery ticket, and the market priced it that way for a reason.
- Selling uncovered (“naked”) calls without understanding the unlimited-loss profile — FINRA flags this risk explicitly (FINRA — Options).
- Holding through earnings without budgeting for the vol crush. Implied volatility collapses the morning after a print, and the option can lose money even when the stock moves in your favor.
- Letting an ITM option expire instead of closing it. Brokers will typically auto-exercise ITM options, which can deliver an unwanted 100-share position into a margin account.
Related concepts — what to learn next
Once the four-number framework feels natural, the next layer is the Greeks — the partial derivatives that show how the premium responds when each input moves. We cover them in Options Greeks Explained: Delta, Gamma, Theta, Vega, Rho. From there, traders typically work outward into spreads (verticals, calendars, butterflies), volatility products (VIX, variance), and structured strategies (covered calls, protective puts, collars).
Sources
- SEC investor.gov — Options glossary definition
- FINRA — Options (investor education)
- Cboe — SPX options contract specifications (European, cash-settled, 100 multiplier)
- Chicago Board Options Exchange — founding and first day of trading (April 26, 1973)
- Options Clearing Corporation — role as central counterparty
- Option (finance) — contract size, intrinsic value, exercise styles
- Black–Scholes model — reference for pricing inputs
Disclosure: This article was produced with AI assistance and reviewed before publication. It is for informational purposes only and is not investment advice.