noun

• (normally archaic) A danger signal or warning.
  "These are the only visible luxuries: the rest is the irreducible minimum of poverty's needs: a wretched bed heaped with all sorts of coverings that have any warmth in them, a draped packing case with a basin and jug on it and a little looking glass over it, a chair and table, the refuse of some suburban kitchen, and an American alarum clock on the shelf above the unused fireplace: the whole lighted with a gas lamp with a penny in the slot meter."

verb

• (archaic) To sound alarums, to sound an alarm.
  "Now o're the one halfe World / Nature ſeemes dead, and wicked Dreames abuſe / The Curtain'd ſleepe: Witchcraft celebrates / Pale Heccats Offrings: and wither'd Murther, / Alarum'd by his Centinell, the Wolfe, / Whoſe howle's his Watch, thus with his ſtealthy pace, / With Tarquins rauiſhing ſides, towards his deſigne / Moues like a Ghoſt."