Faeces is the formal word for the solid waste that leaves the body through the bowel. Doctors write it; scientists write it; reports about rivers and farms write it. At home, British speakers say poo, American speakers say poop, and there are ruder words besides. This spelling with ae is British; American spelling is feces. The word looks plural, and grammar often treats it that way, but you do not say ‘one faece’. Think of it as stuff, not as countable pieces, unless a lab is talking about separate samples. It is a cold word on purpose: it lets people talk about the body without sounding like children or like they are swearing.