Full Idea
Weak alethic relativism holds that while a statement may be true in one framework, it is inexpressible in another. Strong alethic relativism is where a sentence is true relative to one framework, but false relative to another.
Clarification
'Alethic' means 'concerning truth'
Gist of Idea
To say a relative truth is inexpressible in other frameworks is 'weak', while saying it is false is 'strong'
Source
Paul O'Grady (Relativism [2002], Ch.2)
Book Reference
O'Grady,Paul: 'Relativism' [Acumen 2002], p.36
A Reaction
The weak version will be Kuhn's 'incommensurability' of scientific theories, while the strong version will be full Protagorean relativism, saying all beliefs are true.