Relaxation stands quite generally for a release of tension, a return to equilibrium.

In the sciences, the term is used in the following ways:

Relaxation (physics), and more in particular:
Relaxation (NMR), processes by which nuclear magnetization returns to the equilibrium distribution
Dielectric relaxation, the delay in the dielectric constant of a material
Vibrational energy relaxation, the process by which molecules in high energy quantum states return to the Maxwell-Boltzmann distribution
Chemical relaxation methods, related to temperature jump
Relaxation oscillator, a type of electronic oscillatorIn mathematics:

Relaxation (approximation), a technique for transforming hard constraints into easier ones
Relaxation (iterative method), a technique for the numerical solution of equations
Relaxation (extension method), a technique for a natural extension in mathematical optimization or variational problemsIn computer science:

Relaxation (computing), the act of substituting alternative program code during linkingIn physiology, hypnosis, meditation, recreation:

Relaxation (psychology), the emotional state of low tension
Relaxation technique, an activity that helps a person to relax