Coerced or manipulated by any number of social, intellectual, and psycho-emotional methods, to do or accept things you wouldn't have otherwise. It is often an extension of common, everyday persuasion, but goes beyond it to control the way a person thinks and responds in general to limit critical thinking itself, not simply selling a single idea or decision based on presenting information and choice.
The goal of brainwashing is the internalization of the (unrealized or unrecognized) abusive control to where you believe and feel it necessary to enforce someone else's will as your own, unaware of your lack of free will, and usually with an illusion of stronger free will.
It goes beyond Stockholm Syndrome by having fear or doubt with regards to criticism about the authority (person, group and/or system of belief). Rationalizations are given to you to explain away anything contradictory.
The common use of the term implies a blind spot where you cannot see an opposing point of view because of some control or conditioning. Contradictory facts cannot be processed (mental scotomization), or are warped to fit into a form that no longer contradicts (cognitive dissonance).
Unlike simple indoctrination into many religions or philosophies, brainwashing goes beyond education to include reasons and/or phobias why questioning or turning away from it is wrong and people who do not agree are deceived, evil, or just wrong. Some religious and other groups cross the line; many do not.
People can brainwash themselves by being closed-minded, but can be brainwashed intentionally or unintentionally in any abusive relationship, such as a totally dominating partner, parent, or even a company or organization.
This is the primary common use of the word to describe the control process of a cult. However, brainwashing takes diverse forms in terms of tools and degrees of different kinds of control, so saying a group doesn't do brainwashing because of dissimilarities with another group (dressing all the same, or chanting for hours, for example) is invalid -- and a typical defense by people in a cult.
In fact, brainwashing can mean closing the mind when it comes to specific things and not others. Some cults disguise their control by encouraging you to believe anything you want, including staying in your current religion, so long as it does not interfere and you do not question and commit to the group's agenda.
The problem is that the word can be meant or taken as an insult instead of a particular psychological phenomenon. It may be an excuse to explain why someone cannot see your own point of view. Therefore, people who are brainwashed are likely to accuse others and even the whole surrounding society of being brainwashed.
Brainwashing is also a term that cults use to attack religions in general (many of which may have cult characteristics in varying degrees as a natural development tendency of any social group), or religions that are different in basic dogma from their own. Some anti-cult organizations have a cultic mentality themselves and therefore have made it difficult for other groups to advocate and educate about the real psychopathology of brainwashing. In other words, people fall back on the archaic denotation of the word "cult" as "religious sect" and associate brainwashing in terms of belief and not manipulation and control, or blur the distinction between brainwashing and educative indoctrination.
In the end things get really sticky, as modern cults have perfected various techniques and tactics, the most ironic of which is to play the role of an organization promoting free will and fighting against what they conveniently define as brainwashing to not include their own mind control. A common variation of this tactic is declaring the term brainwashing meaningless or even an offensive form of bigotry against religious freedom -- the ultimate and "politically correct" way to silence critics by putting themselves in the role of victim instead of the society they are themselves victimizing.
You must be pretty brainwashed to take out a second mortgage to attend those "self-help" courses.
You only talk about the group you belong to and stopped doing the things you love and spending time with your friends. I know you seem very happy, but we're concerned that you might be brainwashed. Me miss the you that we know.
If you knew you were brainwashed, you wouldn't have been brainwashed very well, right? Trust those who care about you that something is wrong.
I'm not brainwashed! You just don't know the group like I do! They are the most loving, sane, spiritual people I will ever meet and all they want is to save the world! We know things that humanity will only some day understand like we do.
You're the ones who are brainwashed, so I'm not allowed to talk to you anymore because you are interfering with my life progress! ... I mean it's MY choice and it's for my own good since you wont support me in this. Besides, I have to get ready for a mandatory cool aid party next week when the comet comes by. Have any clean pruning shears and a band-aid?
"No you want to do it because yu brainwashed yourself into believing that. Retard."