Singlet Definition:

In NMR spectroscopy, a signal that is comprised of only one peak.

Singlet Explained:

The configuration with the electron spins parallel is at a lower energy than the configuration with paired spins. When the electron spins are paired, their individual spin momentum cancel each other out, and there is no overall spin. There is only one way in which this may be achieved, and hence this configuration is known as a singlet state.