From Halopedia, the Halo wiki
This article is about the military term. For the medal, see Contact (medal).
In military parlance and common idiom, a Contact is an encounter or brief skirmish or fight with an enemy unit, moreover a contact that results or may result in an engagement. Although the term was originally more commonly used by British and Commonwealth soldiers during the 20th and 21st Centuries in war zones, the UNSC Marine Corps and Navy have come to use the term and it can be used to describe either visual, radar, or motion tracker detection of any enemy unit. First Contact can refer to the earliest meeting between two parties previously unknown to one another. In this case, first contact between the Covenant and the UNSC on peaceful terms occurred in 2525.