Touch
The way the keys are struck; shapes loudness and, indirectly, tone color.
What it means
Touch describes, on keyboard instruments, the way a key is depressed: the speed, weight, and motion of finger, hand, and arm involved. On the piano, touch sets the mechanism in motion that throws the hammer against the string; on the organ, it opens a valve that admits wind to the pipe.
Pianists traditionally distinguish many kinds of touch, such as connected (legato), detached (staccato), or held (portato) touch, as well as differences in the hardness or softness with which a note is begun.
Musical use
Touch is the pianist's central means of expression, because once a key is struck the instrument offers no further control over the sounding note: everything meant to shape it must be decided at the moment of touch. Touch speed governs loudness, and the way touch is distributed across several notes shapes phrasing.
Since the nineteenth century, piano pedagogy has argued at length over whether different kinds of touch — a “singing” versus a “pearly” touch, for instance — produce different tone colors at equal volume. Physically this cannot be confirmed: on a freely flying hammer, only its speed at the moment it strikes the string determines loudness, regardless of how that speed was produced. Audible differences do arise indirectly, however, through mechanism noise, pedal use, or tiny timing shifts between the notes of a chord.
Touch should be distinguished from articulation, which governs how notes are joined or separated, while touch describes how a single note is produced. On the organ, the prompt speaking of a pipe is usually called its response rather than its touch.
Performance practice
When practicing, make sure the loudness you want comes from the speed of the finger motion, not from pressing down after contact, since once the hammer is released you can no longer influence the note. Listen critically to whether different touch sensations really change the sound, or whether the difference comes only from timing and pedaling.
Practice legato, staccato, and portato touch separately before combining them, and work on a relaxed hand and arm so that you stay in control even in fast or loud passages instead of tensing up.