A<unk> (Lat.<unk> , an opening or light)  (pl.<unk> a) is an inner space, lining or cavity.
The interior of a vessel within the body, such as the small central space in an artery or vein, or any of their relating vessels through which blood flows.
The inner membrane space of a Thylakoid, endoplasmic reticulum, Golgi apparatus, or microtubule.
Lumen 
Vesicle 
Atom
Cuticle
Sponge
Nucleosol
Clavicle
Boil