单重态（英语：Singlet state），也称单线态，指的是自旋多重性为1的分子。单重态分子的能级在磁场中不裂分，在光谱中只能看到一条能级线。绝大多数分子的基态是单重态。

• 三重态

In quantum mechanics, a singlet (state) is a quantum state of a system in which the total spin of the system (all particles in all directions) adds up to a spin of 0, such that there is only one allowed spin quantum number: 0.

A pair of spin-1/2 particles can be combined to form one of three states of total spin 1 called the triplet, or to a state of spin 0 which is called the singlet. In theoretical physics, a singlet usually refers to a one-dimensional representation (e.g. a particle with vanishing spin). It may also refer to two or more particles prepared in a co-related state, such that the total spin angular momentum of the state is zero. Singlets and other such representations frequently occur in atomic physics and nuclear physics, where one tries to determine the total spin of a collection of particles.