Matter, in the physical sciences, anything that occupies space and has mass. Every physical object—the sea, a star, a book, or a person—is made up of matter. Matter is one of the two basic concepts of the physical sciences, the other being energy (the capacity to do work). Matter and energy can be converted into one another.

Matter exists in three basic states—gas, liquid, and solid. Many scientists also recognize other states of matter, such as plasma (a highly ionized gas) and Bose-Einstein condensate (an extremely cold, dense gas).