What is this equivalence class like set-theoretic construct called?

Question

I have a relation $R$ defined on a set $X$. For any $a\in X$, I have the following sets:

$$[a] = \{x: R(a,x)\}$$

$R$ is symmetric and reflexive, but not necessarily transitive, hence not an equivalence relation. Is there a name for the sets of the form $[a]$?