A facet is one side or aspect of something. If you’re thinking about quitting your day job to become a circus performer, you should first consider every facet of what your new life would be like.
Facet, which is related to the word face, can also refer to one of the flat “faces” of a diamond or other gem. This was the word’s original definition, and it was evidently seen as an apt metaphor for one side of a complex idea. When a problem has many parts to consider (or when it’s just a huge mess), you can describe it as multifaceted.