Facets can refer to different things depending on the context. Here are some definitions based on the search results:

• Geometry: Facets are flat faces on geometric shapes. They reflect the underlying symmetry of the crystal structure and are key to early developments in crystallography.

• Gemstones: Facets are flat surfaces cut into gemstones to improve their appearance by allowing them to reflect light. The angles used for each facet play a crucial role in the outcome of a gem, and they must be carefully adjusted to maximize the optical performance.

• Anatomy: Facets are small, smooth surfaces on a bone or other hard part.

• Search filters: Facets are filters that can be applied after a search to more precisely define the results. They appear on the left after a search and allow users to select multiple facets by hovering or focusing on a facet.

• General: A facet is one side or aspect of something. It can refer to a single part or aspect of a subject or personality, or to one of the flat surfaces of a diamond or other gem.