What does delta letter mean in some ceramics chemical material formula?
Exactly. In your example, it is related to oxygen vacancy (oxygen deficiency) in the crystal lattice.
It can affect the physical properties of the ceramic material. Take the example of this piezoelectric ceramic material:$\ce{Eu_{0.5}Ba_{0.5}TiO_{3-\delta}} $ Oxygen-vacancy induces antiferromagnetism to ferromagnetism transformation.

In general, $\delta$ denotes deviation from the stoichiometry (exces or deficiency) in a chemical formula