The price at which an employee can purchase company stock when exercising their options — set at the fair market value at the time of the grant.
The strike price (also called the exercise price) is the price per share an option holder pays to actually buy the company's stock when they exercise their options. It's set at the company's fair market value (FMV) on the date the options are granted, as determined by a 409A valuation.
The economics of stock options depend on the relationship between the strike price and the eventual stock price. If you're granted options at a $1.00 strike price and the stock is eventually worth $10.00, exercising your options to buy shares and then selling them generates $9.00 of gain per share. If the stock never exceeds the strike price, the options are "underwater" and have no value.
For very early employees, low strike prices (often fractions of a cent or a few cents) are a significant incentive. As the company raises money and its valuation rises, the 409A valuation and thus the strike price for new grants increases. This is why getting in early at a low strike price is valuable: you participate in all the value creation above that low starting point.