Define Sublimation.

Hint: Sublimation is one of the six processes in which a phase change occurs, sublimation is related to gas and solid, it does not include liquid. As sublimation is a phase change process, it can also be reversed, the reverse of sublimation is either de-sublimation or deposition.

Complete answer:
Sublimation is a phase change or transition process in which a substance changes its state directly from solid to gas. Sublimation process does not include liquid phase in it, it directly converts solid into gas by burning or other means.
Sublimation is one of the phase changing processes, there are other five phase changing processes. A phase change process is a process in which a substance goes from one state to another state; this process is either exothermic or endothermic.
The other phase changing processes are melting, freezing, evaporation, condensation and deposition. In a phase changing process a substance gets converted from one phase to another, like from solid to liquid, from gas to liquid, etc.
One phase changing process converts a substance from one phase to another, similarly another process converts that substance from the converted phase back into its initial state. Like melting converts solid into liquid or converts liquid into gas, on the contrary freezing converts gas into liquid or converts liquid into solid.
Thus, for one phase changing process, there is a counter acting phase changing process. For sublimation, the reverse phase changing process is called deposition. In sublimation, solid gets converted into gas and in deposition, gas gets converted into solid.

Note:
A phase cannot only be changed by variance in temperature, but it can also change by variance in either pressure or energy. The factor pressure has to be supplied in more amounts, thus pressure is not considered when it comes to changing phase. Also, there is no matter found till yet whose phase cannot be changed, it may take extreme measures, but a phase of any material can be changed.