I would actually recommend a "hover" instead a pop-up:
Click to Show Information (hover or pop-up)
HTML Code (see tag) (traditional hyperlink)
In short: Write this in your questionnaire:
This is a ((term|In analogy to natural language, where a noun phrase refers to an object and a whole sentence refers to a fact, in mathematical logic, a term denotes a mathematical object and a formula denotes a mathematical fact.)) that needs a definition.