Floating Point Operations per Second
From Unofficial BOINC Wiki
FLOPS is an acronym for Floating Point Operations per Second. This is a figure of merit for computer systems. It measures the number (or quantifies it, if you will) of mathematical operations using arithmetic for Floating Point Numbers that are possible with a specific computer system. Usually the operations that are used as the basis for establishing the FLOPS rating are the addition and subtraction operations since the multiply and divide (and other transcendental functions possible using the Floating Point Unit (FPU) are usually much slower; see Your Mileage May Vary).