STEEM Power (commonly abbreviated SP) is STEEM that has been entered into a smart contract and represents the amount of influence an account has compared to the rest of the network. The more STEEM Power an account has, the higher influence its vote has on posts or comments. STEEM Power also determines an account's weight for witness approval voting.
Accounts are created with a small amount of deposited STEEM in the form of STEEM Power. Content creators have the option to choose post rewards of 100% STEEM Power instead of the default 50% liquid currency (a combination of STEEM and Steem Dollars) and 50% STEEM Power. Users and investors also have the option of converting liquid STEEM into STEEM Power, called powering up.
STEEM Power cannot be transferred or traded directly, but the account owner can choose to withdraw liquid STEEM in weekly payments at a rate of up to 1/13th of the account's total STEEM Power. This is called powering down. The account will lose its voting influence in proportion to the STEEM withdrawn.