Block is an extremely well-known and common still life that was found by John Conway in 1970. In terms of its 4 cells it is tied with tub as the smallest still life, and in terms of its 2×2 bounding box it is outright the smallest.
Due to its small size, its ability to act as an eater, and its simple glider syntheses, it is extremely useful in the construction of large patterns. It can be used as an induction coil as in block and dock, block on table and block on griddle.
The block was one of the patterns described in the original 1970 article by Martin Gardner that introduced the Game of Life to the world.