Forest between which rivers?

[Aiwendil]

It would appear to be the Sirion and the Gelion. From the published Silmarillion, ch. 14:

But south of the Andram, between Sirion and Gelion, was a wild land of tangled forest in which no folk went, save here and there a few Dark Elves wandering; Taur-im-Duinath it was named, the Forest between the Rivers.

Though on the map it appears that it is only bounded by Sirion in its most northern reaches, before Sirion plunges into the sea. One could speculate that it was so named when first encountered by the Sindar. They would have come upon it from the north and so to them it would naturally seem to be "between the rivers"; then it might have been named without thought for how far it extended southward.

Incidentally, on Tolkien's "second Silmarillion map" (found in HoMe XI), it is not clear at all that the forest extends as far south as Christopher's map in the published Silmarillion suggests.