We refer to "unambiguous base class" in this document many times but we never clearly define the concept in this document. I think we can define this concept with the help of [class.member.lookup] p5.3.

[class.member.lookup] p5.3 says

Otherwise, the new S(N, C) is a lookup set with the shared set of declarations and the union of the subobject sets.

We can say

A non-dependent class type B is an unambiguous base class of class C, if and only if C is derived from B and the declaration set and the subobject set of S(B, C) consist of a unique declaration of B and a unique member subobject, respectively.