Acceptance in human psychology is a person's assent to the reality of a situation, recognizing a process or condition (often a negative or uncomfortable situation) without attempting to change it or protest it. The concept is close in meaning to 'acquiescence', derived from the Latin 'acquiēscere' (to find rest in).
The term acceptance is a noun with three different meanings.
The first is the act of taking or receiving something offered. For example, if someone is giving you a gift and you receive it, then you have accepted the gift; therefore, having acceptance.
Another definition of acceptance has to do with positive welcome and belonging; favor and endorsement. For instance, a person could like someone and have acceptance for them due to their approval of that person.
The third description of acceptance is that it can be an act of believing or assenting.
Acceptance - "An express act or implication by conduct that manifests assent to the terms of an offer in a manner invited or required by the offer so that a binding contract is formed. The exercise of power conferred by an offer by performance of some act. The act of a person to whom something is offered of tendered by another, whereby the offered demonstrates through an act invited by the offer an intention of retaining the subject of the offer." (Chirelstein, 2001)
This definition overlaps with the definition of the quality known as toleration. Acceptance and tolerance are not synonyms.
Eckhart Tolle (The Power of Now, etc.) defines acceptance as a "this is it" response to anything occurring in any moment of life. There, strength, peace and serenity are available when one stops struggling to resist, or hang on tightly to what is so in any given moment. What do I have right now? Now what am I experiencing? The point is, can one be sad when one is sad, afraid when afraid, silly when silly, happy when happy, judgmental when judgmental, overthinking when overthinking, serene when serene, etc.
It should be recognized, before any breakdown to types, that acceptance is treating what happens, the actual event which is the outcome of all the combined previous events, as on balance the best outcome. Acceptance typically contains the concept of approval; it is important to note that the psychospiritual use of the term infers a non-judgmental mindset. Acceptance is contrasted with resistance, but that term has strong political and psychoanalytic connotations not applicable in many contexts. By groups and by individuals, acceptance can be of various events and conditions in the world; individuals may also accept elements of their own thoughts, feelings, and personal histories. For example, psychotherapeutic treatment of a person with depression or anxiety could involve fostering acceptance either for whatever personal circumstances may give rise to those feelings or for the feelings themselves. (Psychotherapy could also involve lessening an individual's acceptance of various situations.)