What do you call a '>' relation between two unknowns? [duplicate]

Solution 1:

The example you give, a > 2b, is called an inequality.
In my grade-school math classes we used the term "inequality" to refer even to expressions like a ≥ 2b; language can be messy sometimes.

Equations, inequalities, and expressions like a ∈ ℚ and "b is prime" are all constraints.

Solution 2:

Your question seems to me to provide its own answer. The word relation is often used in this context, it avoids needless debate about whether a ≥ 2b is an inequality or not, and if anyone wants to know exactly what kind of relation they need only look at the mathematical notation.