Well, infinity is an undefined quantity strictly speaking. In Physics, it assumes a relative role. For eg. a distance of 1micron from the centre of a 1 square metre sheet maybe viewed as infinitely large because the sheet expands indeterminately on either side from that distance.

 

In Maths, it follows its regular definition of truly being undefined. In limits, many questions have their limit as "infinity", purists say that the limit is undefined while quite a few say that the limit is infinity.