ictus

noun

1. The pulse.
2. A sudden attack, blow, stroke, or seizure, as in a sunstroke, the sting of an insect, pulsation of an artery, etc.
3. The stress of voice laid upon an accented syllable of a word. Compare arsis.
4. In conducting, the indication of a musical event, most often the beat of the tempo or the entry of a section of the orchestra.