half 
(hăf, häf)
(hăf, häf)n. pl. halves (hăvz, hävz)
1.
a. One of two equal parts that together constitute a whole.
b. One part approximately equal to the remaining part.
2. Informal A 50-cent piece.
3. Sports
a. One of the two playing periods into which certain games are divided.
b. A halfback.
4. Chiefly British A school term; a semester.
5. Half an hour: a half past one.
adj.
1.
a. Being one of two equal parts.
b. Being approximately a half.
2. Partial or incomplete: gave me a half smile.
adv.
Idioms: 1. To the extent of exactly or nearly 50 percent: The tank is half empty.
2. Not completely or sufficiently; partly: only half right.
by half
1. By a considerable extent.
2. By an excessive amount: too clever by half.
by halves
In a reluctant manner; unenthusiastically.
half again as many/much
One-and-a-half times as much or as many; 50 percent more.
in half
Into halves.
not half
Not at all: "Fancy housing? Not half likely, ma'am" (Russell Baker).
not the half of
Only a fraction or a small part of.
[Middle English, from Old English healf; see skel-1 in the Appendix of Indo-European roots.]