A bitmap is a computer picture kept as a grid of tiny dots. Imagine graph paper: each little square is one point of colour, and the computer stores a number for every square. In the oldest kind, that number may be a single bit — just on or off, black or white. Colour pictures use a bundle of bits for each square. Zoom in far enough and you see those squares; that is the giveaway. The other common way to keep a picture is as lines and shapes (a vector image), which stays sharp when you make it bigger. A bitmap does not. Though actually, in everyday talk people say “bitmap” for almost any pixel picture — a photo, a screenshot, even a PNG or JPEG — not only the old .bmp file. Programmers also use it for a grid of bits that is not a picture at all, just packed yes/no flags. Ordinary in computing; you would not use it about a painting on a wall.