Half; a part.
Greek hēmi‑, half.
A hemisphere is half a sphere; a hemicycle is a semicircular shape or structure (adopted for the seating in the European Parliament); hemiplegia (Greek plēgē, stroke) is paralysis of one side of the body. In poetry, a hemistich (Greek stikhos, line of verse) is half a line of verse and in British musical usage a hemidemisemiquaver is a note with the time value of half a demisemiquaver, a sixty-fourth note.
Rather than strictly one half, it can sometimes mean ‘part of’ or ‘a lesser constituent’, as in hemicellulose, substances which occur in the cell walls of plants and are polysaccharides of simpler structure than cellulose, and hemiparasite, a plant which obtains or may obtain part of its food by parasitism but which also photosynthesizes (an example is mistletoe).