Official Aviation Definition
A localizer is the VHF radio transmitter component of an ILS located beyond the far end of the runway. It provides horizontal left-and-right steering guidance to align the approaching aircraft precisely with the runway centerline.
In Simple Words
The part of an ILS radio system that tells the pilot if they are centered, left, or right of the runway centerline.
Real-World Flight Example
The pilot intercepted the localizer beam 10 miles from the runway and turned onto inbound magnetic heading 270.