The initial phase of a played note, or the initial phase of the output of an ADSR envelope generator, when the note’s volume is rising from silence to (typically) its maximum level. Attacks for most acoustic instruments are fairly short and abruptPercussion instruments have very short, spiky attacks, as do guitars played with plectrums, and pianos. Bowed string instruments can have longer attacks depending on playing technique; brass and woodwinds are somewhere in between. Twentieth-century accoustic research discovered that the attack shape and sound is very characteristic to many instruments, to the extent that the instrument is often unidentifiable to the average listerner if the attack is removed, despite the attack's short duration.
Therefore the ability of a synthesizer to create different attack shapes is crucial to both imitating accoustic instruments and to creating novel sounds. For example, a bell will not sound like a bell if it is given a slower attack that would be characteristic of a brass instrument. Setting up an ADSR to produce a very slow attack, combined with a very fast release, will make almost anything sound like a tape being played backwards.