Erase head
A separate head first in the tape path that wipes the old content before the record head lays down the new.
On tape, recording and erasing do not happen in the same place. The erase head sits ahead of the record head and is fed a strong high-frequency alternating current — often from the same oscillator that supplies the bias — leaving a clean, demagnetised surface. AC erase leaves far less residual noise than the cheaper DC erase with a permanent magnet, which some budget cassette recorders settled for.
In multitrack gear the erase head can be split per track, so one track can be re-recorded without touching its neighbours — the basis of studio overdubbing. It also explains why a failing erase mechanism produces "ghost" audio of earlier recordings bleeding under the new material.