A hammer-on is a playing technique that is performed by sharply bringing a fretting-hand finger down onto the fingerboard, causing a note to sound. This technique is the opposite of the pull-off.
Passages in which a large proportion of the notes are performed as hammer-ons and pull-offs are known as legato phrases.
A rapid series of alternating hammer-ons and pull-offs between a single pair of notes is called a trill.