a device that signals the occurrence of some undesirable event
an automatic signal (usually a sound) warning of danger
fear resulting from the awareness of danger
warn or arouse to a sense of danger or call to a state of preparedness; "The empty house alarmed him"; "We alerted the new neighbors to the high rate of burglaries"
march aggressively into another's territory by military force for the purposes of conquest and occupation; "Hitler invaded Poland on September 1, 1939"
penetrate or assault, in a harmful or injurious way; "The cancer had invaded her lungs"
occupy in large numbers or live on a host; "the Kudzu plant infests much of the South and is spreading to the North"
not anticipated; "unanticipated and disconcerting lines of development"- H.W.Glidden; "unforeseen circumstances"; "a virtue unlooked-for in people so full of energy"; "like a bolt out of the blue"