The USAS application suite is a series of diverse and relatively complex mainframe applications written for the Unisys 1100-series, 2200-series, and Clearpath IX environments. These applications are generally intended for use in the airline, transportation, and hospitality industries.

Older USAS applications such as USAS*RES (Reservations System) or USAS*FDC (Flight Data Control) were written originally in Fortran, but elements of various applications were also written in COBOL, Unisys 1100/2200 assembly language (ASM or MASM), and the LINC 4GL .

Most (if not all) USAS applications are written as text-based online transaction systems which are designed for low overhead and fast response times. The environment most commonly used is HVTIP (short for High-Volume TIP).

"USAS" was originally an acronym for Univac Standard Airline Systems, but the product line is now referred to simply as "USAS".