What is the meaning of Completeness?

The state or condition of being complete.

The property of a logical theory that whenever a wff is valid then it must also be a theorem. Symbolically, letting T represent a theory within logic L, this can be represented as the property that whenever ${\displaystyle T\vDash \phi }$ is true, then ${\displaystyle T\vdash \phi }$ must also be true, for any wff φ of logic L.

Source: wiktionary.org