The answer to this question depends on whether you're talking about a literary analysis, or an analysis for data modeling purposes, which is what the post is actually asking for.
The Wikipedia page linked in the OP gives the literary definition: a prequel is a sequel whose timeline is earlier than the original work. In this sense, Fate/Zero is a sequel to Unlimited Blade Works because it expands on the original work, the VN which had UBW as one story route. In this sense, Fate/Zero is also a prequel to UBW because it takes place earlier in the timeline than UBW.
But you don't really need the literary definition: you want to model relationships between series in a database of some sort. Using a literary definition for this will be a nightmare, because literary definitions are often imprecise, sometimes vague, occasionally contradictory, and usually fuzzy around the borders. The Wikipedia page gives some good Western examples of this; I'll here list some anime examples:
- Deen Fate/Stay Night and Unlimited Blade Works take place in the same universe at exactly the same time, both being adaptations of story routes from the Fate/Stay Night visual novel. You can't really say that these shows are either prequels or sequels to each other in the usual sense. In the visual novel, Unlimited Blade Works does expand on the Fate arc (adapted in Deen Fate/Stay Night), but it occurs neither before nor after Fate; it occurs instead of Fate.
- Yuru Yuri and Rewrite happen in the same world, at the same school, and involve several of the same characters, but it's vague where they actually sit relative to each other in the timeline.
- Owarimonogatari, despite being the most recent series, takes place after Nisemonogatari and concurrently with the first arc of Second Season.
- If you include manga, then all the Madoka manga except the first one (which directly adapts the anime) occur in alternate timelines. You can sort of order them because Homura experiences them in a certain order, but it's usually impossible to tell where they go relative to each other. Also, The Different Story Volume 1 is a prequel—its events take place before the anime and hold for all the alternate timelines—but Volumes 2 and 3 are yet another alternate timeline.
I would just throw out the prequel/sequel jargon altogether. It's somewhat useful for literary analysis, but it's too coarse-grained to make for good data modeling. There are two important things with anime: the in-universe chronology, and the order in which to take in the various parts of a franchise. The second one usually corresponds to production order, though it gets weird when you don't have the original work to fall back on (as is the case with the Fate series). The in-universe chronology is clear: Fate/Zero is before Unlimited Blade Works. It's also clear, when you include the original work, that you were meant to experience the visual novel, then the Fate/Zero novel or anime, then the Unlimited Blade Works anime. When you take the visual novel out of the equation, there's no clear answer, as this old question of mine shows. But you will drastically simplify the data modeling by treating these two separate things as separate pieces of data.
