In English, to be inert is to be in a state of doing little or nothing.
In chemistry, the term inert is used to describe something that is not chemically active. The noble gases were described as being inert because they did not react with the other elements or themselves. It is now understood that the reason that inert gases are completely inert to basic chemical reactions (such as combustion, for example), is because their outer valence shell is completely filled with electrons. With a filled outer valence shell, an inert atom is not easily able to acquire or lose an electron, and is therefore not able to participate in any chemical reactions. For inert atoms or molecules, a lot of energy is involved before it can combine with other elements to form compounds. A high temperature and pressure is necessary, and sometimes requires the presence of a catalyst.
For example, elemental nitrogen is inert under standard room conditions and exists as a diatomic molecule, N2. The inertness of nitrogen is due to the presence of the very strong triple covalent bond in the N2 molecule.
In the branch of mathematics known as algebraic number theory, a prime ideal is said to be inert if it is still prime when considered in an extension field. Such a prime might have instead split as a product of other prime ideals, but by being inert it remains essentially unchanged.
In weaponry, an inert munition is one in which all energetic material such as primer, fuze, and explosive or incendiary fill have been removed or rendered harmless. Inert munitions are used in military training and are collected in museums and by private individuals.