Definition

Approach lighting system (ALS). Provides lights that will penetrate the atmosphere far enough from touchdown to give directional, distance, and glidepath information for safe transition from instrument to visual flight.

Source: US FAA Pilot's Handbook of Aeronautical Knowledge

Description

An approach lighting system (ALS) is a configuration of signal lights placed symmetrically on both sides of the runway extended centreline, starting at the landing threshold and extending outward into the approach zone. This system provides visual information about runway alignment, height perception, roll guidance, horizon references and limited distance-to-go information. 

The fillowing types of ALS are defined by ICAO: