[Q] Erfi - what is it?

[Michael C. Herron]

I used mathematica to solve an integral that I could not myself do
analytically. The output contained a function Erfi[]. What is
Erfi[]? I am familiar with the error function and its complement
(Erf[] and Erfc[] in mathematica), but I do not understand Erfi[].
The function does not appear to be in the manuals that I have seen,
either.

If someone could translate Erfi[] or provide a reference, I would
appreciate it.

Thanks.

Michael