entailment

entailment:
from w3c RDF Semantics W3C Recommendation 10 February 2004

• Entail (v.), entailment (n.). A semantic relationship between expressions which holds whenever the truth of the first guarantees the truth of the second. Equivalently, whenever it is logically impossible for the first expression to be true and the second one false. Equivalently, when any interpretation which satisfies the first also satisfies the second. (Also used between a set of expressions and an expression.)
• Equivalent (prep., with to) True under exactly the same conditions; making identical claims about the world, when asserted. Entails and is entailed by.

vocabulary entailment:
OWL is defined as an extension to RDF in the form of a vocabulary entailment, i.e., the syntax of OWL is the syntax of RDF and the semantics of OWL are an extension of the semantics of RDF.

may be I should look into the RDF/owl semantics again. for a complete understanding on ontology entailment