For me, the concept of infinity is best represented in terms of another number, such as in writing domains or drawing number lines. Unlike the standard form of a domain, however, the number line simply starts at a point (let's say x > 0) and austerely points in the direction of positive infinity. There is no such actual number as infinity (though it IS used in math, and is definitely useful) because it is not part of the series (a number by definition is part of a series) of numbers ranging from one to infinity. The "number" infinity is more of an idea and a (very useful) tool than an actual number.