onehalf

One half is the fractional part that results when a whole is divided into two equal parts. It is written as the fraction 1/2, and it equals 0.5 in decimal notation. As a percentage, one half is 50 percent. In mathematical terms, 1/2 is the reciprocal of 2, meaning multiplying a quantity by 1/2 halves it.

One half appears across arithmetic and algebra. Adding two halves yields a whole, and multiplying a quantity by 1/2 scales it down by 50 percent. In binary arithmetic, 1/2 is represented as 0.1 in base 2. In probability, the chance of a fair coin landing heads is 1/2.

In geometry and everyday use, one half is often described in relation to halves of objects or figures. A semicircle, for example, is half of a circle. The concept is also common in cooking and measurements, where a recipe might call for half a cup or half a teaspoon.

Etymology and spelling notes indicate that the term originates from the Old English healf. In modern usage, "one half" or "one-half" is standard, with the hyphen used when the phrase functions as a compound adjective. The closed form "onehalf" is not generally standard, though it may appear in some informal or specialized contexts.