FLOP - exact definition
Just to add to whychmaster's reply, in my experience, on a GPU, a floating-point operation, when used in quoting FLOPS (floating-point operations per second) benchmark figures will refer to Addition (equiv subtraction) and Multiplication.
The other standard IEEE operators, division + sqrt, are more computationally expensive and so unlikely to be included in the figure. (Note that, there is often HW support in GPUs for accelerating operations like 1/x, 1/sqrt(x) etc, perhaps to make them as (or nearly as) fast as muls &amp; adds)
Having said that, it is very likely that a GPU will actually implement a Fused Multipy-Add operation, FMAD, i.e. do  A * B + C in &quot;one step&quot; as this is

more accurate than doing separate multiply and add instructions
is faster than separate ops
is about the same silicon cost
is a very common building block of CG operations (e.g dot products, convolutions)

I suspect that in quoted figures, though, an FMAD would be considered 2 operations.