atom at·om (āt'əm)
A unit of matter, the smallest unit of an element, having all the characteristics of that element and consisting of a dense, central, positively charged nucleus surrounded by a system of electrons. The entire structure has an approximate diameter of 10-8 centimeter and characteristically remains undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons.
This unit regarded as a source of nuclear energy.
A part or particle considered to be an irreducible constituent of a specified system.
The irreducible, indestructible material unit postulated by ancient atomism.
An extremely small part, quantity, or amount.