I’m favour of using the terminology “stress emphasis” and “strong importance”; maintaining consistency with HTML5 makes sense since that is the primary output format.
Definitions from the HTML5 spec:
http://www.w3.org/TR/html5/text-level-semantics.html#the-em-element
http://www.w3.org/TR/html5/text-level-semantics.html#the-strong-element