"Zero conditional" refers to conditional sentences that express a factual implication, rather than describing a hypothetical situation or potential future circumstance. The term is used particularly when both clauses are in the present tense; however such sentences can be formulated with a variety of tenses/moods, as appropriate to the situation:
If you don't eat for a long time, you become hungry.
If the alarm went off, there's a fire somewhere in the building.
If you are going to sit an exam tomorrow, go to bed early tonight!