1. The projection of the quantum-mechanical spin of a particle onto the direction of its motion. The helicity of a particle is said to be right-handed if the direction of the spin is the same as the direction of motion and left-handed if the direction of spin is opposite to the direction of motion. All the observed neutrinos have left-handed helicity.

2. A quantity that characterizes the topology of vortex motion (see vortex dynamics) in fluids. Knot theory is extensively used in discussions of helicity in fluid mechanics and magnetohydrodynamics.

3. A measure of the extent to which magnetic field lines wrap around each other. Magnetic helicity is characterized by topology and has been found to be useful in plasma physics and the study of the magnetic field lines in the earth and sun.