A bit is a single digit in the binary number system, and the smallest unit of information that a digital computer can deal with. It’s a single item that is either “on” (a numerical value of 1) or “off” (0).
Everything (and I do mean everything) that digital computers do and deal with just use collections of bits. Computer programs? Represented using bits. Photographs? Bits. Videos? Lots and lots of bits. This definition? A collection of bits stored on a computer somewhere.