To stop a thing before it completes. Cancelling leaves the intention on record and the act undone.
Latin cancellare (to make lattice-shaped, to cross out with crossed lines), from cancelli (crossbars, a lattice), diminutive of cancer (a crab, or a lattice) — the same root behind 'chancellor.'
To stop a thing before it completes — the intention stays on the record even after the act itself is undone.