Strike Price

The price at which an option holder can buy (for a call) or sell (for a put) the underlying asset.

Example

A call with a 105 strike on a share trading at 100 is 5 below being worth exercising.

The strike, not the current price, decides whether an option has any value at expiry.

Call Option — A contract giving the buyer the right, but not the obligation, to buy an asset at a set price before or on a set date.
Put Option — A contract giving the buyer the right, but not the obligation, to sell an asset at a set price before or on a set date.
Intrinsic Value (Option) — The amount an option would be worth if exercised immediately, and zero if exercising would not pay.

A contract about a price, not a piece of anything — An option or future is an agreement whose value depends on another asset. You own a right or an obligation with an expiry date, not a share of a business. (10 min)
Big percentages on a small bet, and most bets expire worthless — Option payoffs are lopsided: a bought option can multiply its premium or lose all of it, and the price usually has to move a long way just to break even. (12 min)
You pay for intrinsic value, time and uncertainty - and time runs out — An option's price has two parts: what it would be worth if exercised now, and extra value for the time and volatility left. That second part shrinks to zero by expiry. (12 min)