R (1.0 + epsilon) not equal to R (1.0) where the function R maps a real number to a representation that uses a finite amount of computer memory (limited precision).
It is often used as a stopping criteria for algorithms since a change less than epsilon would not result in the change in a floating point value as represented in the computer's memory.
Epsilon for an 8 byte IEEE floating point number is about 2.2 x 10-16 and is defined as DBL_EPSILON in the C standard library in the header file "float.h".