Shares under option
An option award allows the holder to acquire a specified number of shares for a specified price. The specified price may be called the strike price or the exercise price. The shares under option is the specified number of shares. The holder may exercise the option in whole or or in part. For example, if there are 10,000 shares under option and the vesting requirements are satisfied, the holder may exercise their right to acquire 2,000 shares today and at various later times (but no later than the expiration date) exercise their right to acquire the remaining 8,000 shares.
Sometimes, the number of shares under option is call the grant size or the grant amount.