dfn is the definition of something.
<p>The prefix cardio- means <dfn>of the heart</dfn>.</p>
As far as I can see in the list of HTML 4 elements, nothing specifically suits the bill. This leaves you with a few options:
<span class="scientific">cardio</span>
Semantics are added by the class, and therefore, this is perhaps the most correct way, technically. However, this has a flaw that without your CSS, it will in no way be different from the surrounding text. Another option might be: / me prepares for duck for cover
<i class="scientific">cardio</i>
, , verboten, <i>, , , <span>, , . , class.