Shortbread is a type of unleavened biscuit (cookie) which is traditionally made from one part white sugar, two parts butter, and three parts oatmeal flour. The use of plain white (wheat) flour is common today, and other ingredients like ground rice or cornflour are sometimes added to alter the texture.
Shortbread is so named because of its crumbly texture (from an old meaning of the word short). The cause of this texture is its high fat content, provided by the butter. The short or crumbly texture is a result of the fact that the fat inhibits the formation of long protein (gluten) strands. The related word “shortening” refers to any fat that may be added to produce a short (crumbly) texture.
Shortbread is different from shortcake, which can be similar to shortbread, but which can be made using vegetable fat instead of butter and always uses a chemical leavening agent such as baking powder, which gives it a different texture.
Shortbread biscuits are often associated with normal egg-based biscuits, but they hold their shape under pressure, making them ideal for packed meals.