I am guessing Obsidian is interpreting whatever you put between angle brackets as HTML.
Iirc, Markdown is supposed to include any HTML in a document. So you can old something with <strong>text</strong> just as well as **text**.
If I’m right, then anything that is valid HTML — including <definition> ... </definition> — will style something in your note. Anything that isn’t recognized as HTML should probably be ignored.