Definition of blind
In no-limit Texas Hold'em, the most popular poker variant, and in many other poker variants, there are two types of blinds, the small blind and the big blind. The blind is a forced bet before the start of each hand. On a 10-player table, for example, the small and big blinds will move clockwise on every hand. The blinds (small and big) are the two positions to the left of the button (dealer). So, if in a hand Player 1 is the button, Player 2 will be small blind and Player 3 big blind. In the next hand, Player 2 will be button while Player 3 will be small blind and Player 4 will be big blind.
Clarification of the term "blind
When we talk about "the blinds", we usually mean the small and big blinds (the players in the blinds). When we talk about a blind, we usually mean the size of the big blind. On a table with a 5/10 limit (small blind 5 and big blind 10), if I raise from 3 blinds, I don't raise to 15 (3x the small blind), but to 30 (3x the big blind).