What does the letter e stand for in the formula for current in an RL circuit?

[complexc25]

I know I(t) = Io [1 - e-t/t ] is the formula to calculate the current in an RL circuit, however i have no clue what "e" stands for. Can someone give me an insight?

 

[Mapes]

e is the "en.wikipedia.org/wiki/Exponential_function"[/URL]: here, it means $\mathrm{exp}(-t/\tau)$, where $t$ is time and $\tau$ is a characteristic time constant.