A relation R on a set A is a partial order if it is reflexive, anti-symmetric, and transitive. The notation a ≼ b is used to express aRb, reflecting the fact that a partial order acts like the ≤ operator on the elements of A.
The expression a ≼ b is read “a is at most b.”
The difference between the ≼ and the ≤ symbols is the slight curve in the ≼ symbol. The domain, along with a partial order defined on it, is denoted (A, ≼) and is called a “partially ordered set” or “poset.”