(countable) An addition or annex to a group, set, or total.

(uncountable) The act of including, i.e. adding or annexing, (something) to a group, set, or total.

(countable) Anything foreign that is included in a material,

(countable, mineralogy) Any material that is trapped inside a mineral during its formation, as a defect in a precious stone.

(cytology) A nuclear or cytoplasmic aggregate of stainable substances.

(histology) An object completely inside a tissue, such as epidermal inclusion cyst, a cyst in the epidermis.

(mathematics) A mapping where the domain is a subset of the image.