Q:

What does EPS mean in C?

I have the following code snippet:
if (ABS(p43.x)  < EPS && ABS(p43.y)  < EPS && ABS(p43.z)  < EPS) return(FALSE);

Which I'm trying to convert to C#.  What does "EPS" mean?
This code is from http://local.wasp.uwa.edu.au/~pbourke/geometry/lineline3d/

A:

It's going to be some form of epsilon to determine whether the number is "small enough to be insignificant". The exact value looks like it's being #defined somewhere in this case.

A:

EPS is epsilon.  The "close-enough" factor.
The question is "is the absolute value close enough?"  Where "close enough" is some small number, often something like 1.0E-3.
Depending on how the algorithm converges on the answer, the performance may depend on the size of EPS.  Be careful of making EPS too small, because your process could run for hours (or centuries) and not produce a really usable answer.
In this case -- where there's no loop -- the EPS is used because floating point numbers accumulate small errors during multiplication.  You can't simply say
a == b

And have it be true in general.  So instead we always say
abs( a-b ) <= EPS

