Perpetuity

A perpetuity is a constant stream of identical cash flows with no end. The formula for determining the Present Value of a perpetuity is as follows:

$PV$ $=$ ${\frac {C_{1}}{(1+r)^{1}}}+{\frac {C_{2}}{(1+r)^{2}}}+{\frac {C_{3}}{(1+r)^{3}}}+...={\frac {C}{r}}$

Source: http://www.investopedia.com/terms/p/perpetuity.asp

In Principles of Corporate Finance the perpetuity is defined as an Annuity in which the periodic payments begin on a fixed date and continue indefinitely.