Personally I prefer the term "simple index" to "single field index". You're welcome to pick the term that you like, but let me give you my reasoning:

* The term "simple index" contrasts well with the terms "compound index" and "multi index"
* The sentence "single field indexes based on the value of a single field" just reads awkwardly because it's sort of a tautology
* The term "simple" is nice because it gives a hint as to how easy these are to use in Rethink