What is a perturbation and how does it affect a system in equilibrium?

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Perturbation-the ability of an unperturb system to remain the same when a perturb system is added to it. Can't really understand this?
 
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It would be really helpful, if you were able to formulate your questions in a comprehensive manner and your statements in a non-tautological and non-repetitve way.

A perturbation (of a system found in an equilibrium state) is simply a (generally) small interaction to which the object(s) under observation is(are) subject to.

In a quantum mechanical setting, a perturbation is described by a new term added to a Hamiltonian operator whose spectral problem is completely solved.
 

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