noun

• (informal) Action of spotting or seeking out a bird, especially a rare one.
• (farriery) A stick with a hole in one end through which passes a loop, which can be drawn tightly over the upper lip or an ear of a horse and twisted to keep the animal quiet during minor surgery.
  "THE TWITCH is a short stick of strong ash, about the size of a mopstick, with a hole pierced near the end, through which is passed a piece of strong but small cord, and tied in a loop large enough to admit the open hand freely."
• (physiology) A brief, contractile response of a skeletal muscle elicited by a single maximal volley of impulses in the neurons supplying it.
• (mining) The sudden narrowing almost to nothing of a vein of ore.

verb

• (intransitive) To perform a twitch; spasm.
  "His fingers were nervously twitching."
• (transitive) To cause to twitch; spasm.
  "Their feet padded softly on the ground, and they crept quite close to him, twitching their noses..."
• (transitive) To jerk sharply and briefly.
  "to twitch somebody's sleeve for attention"
• (obsolete) To exert oneself.