here is a definition from Feminist philosopher and cultural critic, bell hooks:

"I have wanted [friends curious or skeptical about feminism] to have an answer to the question "what is feminism?" that is rooted in neither fear or fantasy. I have wanted them to have this simple definition to read again and again so they know: "Feminism is a movement to end sexism, sexist exploitation, and oppression." I love this definition... I love it because it so clearly states that the movement is not about being anti-male. It makes clear that the problem is sexism. And this clarity helps us remember that all of us, female and male, have been socialized from birth to accept sexist thought and action. As a consequence, females can be just as sexist as men. And while that does not excuse or justify male domination, it does mean that it would be naïve and wrongminded for feminist thinkers to see the movement as simplistically being for women against men. To end patriarchy (another way of naming institutionalized sexism) we need to be clear that we are all participants in perpetuating sexism until we change our minds and hearts, until we let go of sexist thought and action and replace it with feminist thought and action"

• Wikipedia on Patriarchy
• but also, Wikipedia AS AN EXAMPLE of patriarchy itself 
• "Anyone can edit Wikipedia, but over 80% of Wikipedia’s editors are young, white, child-free men, which means that their perspective is what largely dominates how information is organized, framed and written. There’s nothing inherently wrong with a young, white, child-free man’s perspective, of course– it’s just that there are tons of other perspectives in the world that should influence how a story gets told. "

--- taken from bell hooks' book Feminism Is For Everybody

---- check out rest of intro and Chapter 1 for a good sample. the chapters are very short.