Good entry already exists here: http://en.wikipedia.org/wiki/Conservative_extension (2AO6)
Example: (2AOH)
Plain English: (2AP0)
You have a FormalOntology for numbers. You extend your ontology by introducing a FormalOntology 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). (2XMQ)
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. (2AO8)
Formal Example (2AP1)
... (2AP2)