A spin is a single play of a slot. Your stake is deducted, the RNG selects the outcome, and the reels animate to show it before any win is paid.
The result is fixed the moment the spin starts. Stopping, slowing or watching the reels has no effect, and the previous spin has no influence on the next one.
A spin that triggers cascades, respins or a feature can run on for some time before it settles, all as one game round. In Great Britain an online slot spin must take at least 2.5 seconds as a game cycle, so no online slot can run faster than 1,440 spins an hour. On fruit machines the equivalent is a "play", started by a button rather than the lever that gave the one-armed bandit its name.