Urban dictionary isn’t really trustworthy, you have to be prepared to get trolled.

Wiktionary is a sister project of Wikipedia and is to something like the Oxford English Dictionary or Merriam-Webster what Wikipedia is to Britannica.

Re: Urban Dictionary: sort of, but the slang is more curated. The inclusion criteria on Wiktionary are pretty lax, and it’s way more descriptivist than prescriptivist. For example, we have the word “hitstun”, which is fighting game jargon you won’t find in a major dictionary.

A major aspect Wiktionary has that other major dictionaries don’t is that it’s an everything-to-English dictionary (or everything-to-whatever-language, e.g. fr.wiktionary.org is an everything-to-French dictionary). So as a very, very extreme example, this is the English Wiktionary entry for ‘no’, which we have entries for in 91 languages. We even keep track of languages like Akkadian, Klingon, Proto-Indo-European, American Sign Language (ASL), and so on. (In total, we have definitions from over 5100 languages.)

It’s also way more flexible than a standard dictionary. For example, the French entry for Sagittarius (“Sagittaire”) has an image of Sagittarius and a ‘See also’ table of every other Zodiac sign in French. We also, of course, often have links back to Wikipedia. Likewise, (like the Oxford English Dictionary) we keep track of etymology, so for example, you can learn that the word “slogan” comes from Scottish Gaelic for “battle cry”. We keep track of rhymes, anagrams, derived terms, etc. too. And definitions will (ideally) also have real quotations so you can see how they’re used in a sentence.

Probably the main lacking aspect right now is the thesaurus, which exists but is undercooked.

TL;DR: Yes, it can replace a normal dictionary and then some. I don’t think it can replace a normal thesaurus yet for most people.