the name "eCash" predates bitcoin. It's a cryptographic scheme for private online transactions. Has been around since 1982. eCash can represent anything. In the case of Cashu, it represents bitcoin held by the custodian.

Here is a post made by Hal Finney discussing eCash in 1993: https://web.archive.org/web/20140410150152/http://finney.org/~hal/chcash2.html