Subtype is not part of the common jargon in C++. The definition in Wikipedia (thanks Chad) is quite broad and in C++ could represent multiple different things, including but not limited to inheritance. For example, all iterator types from a given category and pointers could be subtypes of the iterator concept as they can be substituted in templates that require that concept (including the standard library algorithms).
I would use derived in general, other alternative words for the same (in different languages) could include extends (type A extends B) or inherits.