/ˈästēəm/
Noun, pl. ostia
1. A small opening or orifice, as in a body organ or passage. (wiktionary.org)
2. a. Any of the small openings or pores in a sponge. (wiktionary.org)
b. Each of a number of pores in the wall of a sponge, through which water is drawn in. (Google Dictionary)
3. A mouthlike opening in a bodily part (as a fallopian tube or a blood vessel). (merriam-webster.com)