Search covers every wiki category at once, so you do not have to know whether the thing you are looking for is a weapon, an attachment, a material or a blueprint. Results come back grouped by category.
Understanding roughly how the matching works turns most failed searches into successful ones, because nearly all of them fail for the same two or three reasons.
How matching works
Every word you type has to match. Search is an AND, not an OR. Typing imperial armor finds entries matching both words, not everything matching either. Adding words narrows results, it never widens them, so when you get nothing back, remove a word rather than adding one.
Words match from the start. Each word is matched as a prefix, so ato finds Atrox and imp finds Imperial. You do not have to finish typing a name, but you do have to start it correctly, and there is no matching from the middle: trox will not find Atrox.
Names rank above descriptions. Both are searched, and an entry whose name matches is ranked above one that merely mentions the term, which is why the thing you actually wanted is usually at the top of its group rather than buried.
Hyphens are treated as spaces. Searching mind-force, mind force or mindforce behaves the same way for hyphenated names. Punctuation you type is stripped, so a stray colon or quote will not break the search or throw an error.
Two characters minimum. A single character returns an error rather than results, because a one-letter prefix would match most of the database.
When nothing matches exactly
If the prefix search finds nothing at all, search falls back to fuzzy matching on names, which tolerates typos and small misspellings. atroxx still finds Atrox.
The fallback only runs when the normal search returns zero results. That is worth knowing, because it means a search that returns one bad result will not be rescued by it. If you get a single irrelevant hit, one of your words is matching something you did not intend; drop that word and try again.
Why you might not find something
It is not in the wiki yet. Search only covers entries that exist. The wiki is maintained by volunteers and it is not complete, particularly in categories that were never imported in bulk. See data health and completeness for how the gaps are tracked and worked through.
You searched a property rather than a name. Search matches names and descriptions, not numeric fields. There is no way to search for "weapons above 100 DPS" from the search box; use the filters on a category's list page for that.
It is an armour part. Individual armour pieces are deliberately excluded from search results, since a single armour set would otherwise flood a search with seven near-identical rows. Search for the set and open it to reach the parts.
Finding things faster
Go through the category list when you know the category. List pages have filters and sorting built for their category, and they will beat search for anything you want to narrow by numbers rather than by name.
Follow the links between entries. Entries are cross-linked, so a creature lists what it drops and a blueprint lists what it needs. Landing on a roughly-right entry and following one link is usually quicker than composing a better search.
Search covers documentation too. These pages are searchable from the same box, so a question about how something works can be answered from the same place you look things up.
Newly added entries
New and edited entries are indexed as they are saved, so they are searchable immediately. If an entry exists but does not come back from search at all, that is a genuine indexing fault worth reporting, rather than something you can fix by rephrasing.