Equisetum arvense strobilus cut open to reveal sporangia
A sporangium (pl: sporangia)  is an enclosure in which spores are formed. It can be a single cell or multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle. Sporangia can produce asexual spores by mitosis, but in nearly all land plants and many fungi, sporangia are the site of meiosis, and produce genetically unique haploid spores.
In flowering plants the sporangium tissue (except the epidermis) that produces the female spores is also called the nucellus.