Both :) the standard unit Of time is a dit. A dah is 3 dits long. 1 dit between each element (so s is ... But 5 dits long because its on, space, on, space, on). 3 dits between letters. 7 dits between words.

When people make the dits too long or the dahs too short (c vs y), or run characters together (ee or i?), it gets hard to copy correctly.

From what I remember reading, when they sent code over telegraph wires, dit and dah were distinguished by the time between clicks. Each one required two clicks to send, but the clicks were closer together for a dit.

You could probably Google it to be sure.