A single deliberate press. Also the sound of one — the interaction named for the noise it made.
Middle English clicken, likely imitative of the sound itself and reinforced by Dutch klikken; the word sounds like the thing it names, in the same pattern as 'bubble.'
The interaction borrowed its name from a byproduct — the sound was never the point of the press, only evidence that the press had happened, and the word kept that evidence's name long after the mechanisms that made the actual noise disappeared.