An active episode of a chronic or recurrent disease.
(music) The onset of a musical note, particularly with respect to the strength (and duration) of that onset.
(audio) The amount of time it takes for the volume of an audiosignal to go from zero to maximum level (e.g. an audio waveform representing a snare drum hit would feature a very fast attack, whereas that of a wave washing to shore would feature a slow attack).
(transitive) To aggressivelychallenge a person, idea, etc., with words (particularly in newspaper headlines, because it typesets into less space than "criticize" or similar).
(transitive) To begin to affect; to act upon injuriously or destructively; to begin to decompose or waste.
(transitive) To deal with something in a direct way; to set to work upon.