Block Height
The position of a block in the chain, counted from the first block, which sits at height zero.
Height is a simple counter, and protocols use it instead of clock time because every node agrees on it exactly. Upgrades are scheduled to activate at a stated height, and issuance schedules step down at fixed heights rather than on calendar dates. The number of confirmations a transaction has is the current height minus the height of the block that contained it. Readers meet height in explorers, in upgrade announcements, and in exchange deposit policies that require a set number of confirmations.
In practice
Bitcoin's block subsidy halves at fixed heights, every 210,000 blocks, rather than on a particular date.
The common misunderstanding
Height is not a clock; blocks arrive irregularly, so a given height corresponds only approximately to a moment in time.