Good entry already exists here: http://en.wikipedia.org/wiki/Conservative_extension
You have a Formal Ontology for numbers. You extend your ontology by introducing a Formal Ontology about elephants. You can now talk about 1 elephant or 5 elephants etc. If you now have one ontology that includes both numbers and elephants, we say that you have conservatively extended your ontology of numbers (by adding an ontology for elephants).
Let us call the number ontology T1, and the combined number and elephant ontology, T2. If we think of a Conservative Extension as a binary relation between two formal ontologies, then we can say that T2 conservative extends T1.