A key used to raise, or throw back, the latch of a door, esp. a night latch.

key for raising or drawing back a latch or opening an outside door

A latchkey is English vernacular for a door key, usually the key that opens the back door. Leaving a door 'on the latch' means to leave it unlocked. The latch itself refers to the old mechanism for securing a door, where a small metal or wooden bar dropped into a slot on the doorframe.