The current price of the stock or index, the base from which the contract price is derived.
What this price is
The underlying price (Underlying Price · S) is the price of the stock or index the contract is built on. It is the letter S in every pricing formula. A contract is a shadow of this asset: all its Greeks and meaning are measured from its position and distance from it.
The reference everything is measured against
Where S sits relative to the strike classifies the contract (in, at or out of the money) and sets its intrinsic value. The distance sets Delta and Gamma. Its movement is what every strategy bets on or against. A $1 change in S shakes the whole chain, each contract by its own sensitivity.
Which price exactly?
During the session: the last trade, or the midpoint for precise work. Outside it the answer branches: an official close, extended-hours trading that can jump, and special settlement prices for some indexes. Precision about which S you use settles many calculation disputes that look like mystery.
Hypothetical example
Two calculators price the same contract and disagree. One used the last extended-hours trade ($101.30) and the other the session close ($100.85). Half a dollar in S moved the contract from out of the money to in the money on paper. The disagreement is in the input, not the math.
Common mistake
Comparing calculations built on two different or old values of S: every number on the page ages as it ages. That is why the platform stamps each number with its measurement time.
Why it is stamped with its measurement time
Every calculation in the options chain is derived from it: the contract's intrinsic value, whether it is in the money, its delta and its expected move. A price that is minutes old makes every number built on it stale, and a price taken from thin extended-hours trading liquidity can sit far from the official close, enough to flip the contract's category.
Where to see it on the platform
Every contract tool shows the reference S with its time. The calculator makes it the first input and pulls it live with the ticker button.
Every options tool shows it stamped with its time, and the calculator pulls it live with the ticker button, so you never type it by hand.
