The mechanical device that moves something , usually read/write head.

So, despite not really knowing how to play the piano and not knowing how to read or write music, Cicoria got himself a piano and promptly began to play everything he heard in his head.

The preamp has much more tracks going to the heads (right side) than to the HDA (left side), it's because HDD can work only with one "head" (pair of read an write elements) at a time.