a bit
/ bit /
Imagine the smallest possible question a machine can answer: yes or no, on or off. A bit is exactly that — one tiny piece of information that can only ever be one of two values. We usually write those two values as 0 and 1, but they could just as well be 'off' and 'on', 'false' and 'true', or 'no current' and 'current'. Everything a computer stores or computes is built up out of nothing but these two-way choices.
The name is short for 'binary digit'. Just as a decimal digit picks one of ten symbols (0 through 9), a bit picks one of two. One bit on its own carries very little, but bits combine: with 2 bits you can make 4 different patterns (00, 01, 10, 11), with 3 bits you can make 8, and in general n bits give you 2 to the power of n distinct patterns. That doubling is why even small groups of bits can name surprisingly many things.
A bit is an amount of information, not a thing you can point at directly — a single transistor, a magnetic spot, or a pulse of light might physically hold one bit, but the idea is independent of the hardware. The honest caveat: a lone bit by itself means nothing until you decide what it stands for. The same 1 might mean 'the switch is on', 'the number is odd', or 'this option is enabled' — meaning is something we assign, not something the bit carries on its own.
A light switch is one bit: 0 = off, 1 = on. Eight such switches side by side make one byte, able to show 2^8 = 256 different patterns.
One bit answers one yes/no question; bits gang up to count.
Beware the lowercase/uppercase b: a 'bit' (lowercase b, as in Mbps) is one binary digit, while a 'byte' (uppercase B, as in MB) is eight bits, so they differ by a factor of eight.