The advocacy of women`s rights on the grounds of sexual equality (OED) (seen via the finallyfeminism101 blog)
Feminism is a collection of movements aimed at defining, establishing, and defending equal political, economic, and social rights for women. (Wiki)
Feminism: 1. the theory of the political, economic, and social equality of the sexes / 2. organized activity on behalf of women's rights and interests. (Merriam-Webster’s Collegiate Dictionary)
I myself have never been able to find out what feminism is; I only know that people call me a feminist whenever I express sentiments that differentiate me from a doormat or a prostitute. (Rebecca West)
Feminism is the struggle to end sexist oppression. (bell hooks)
Feminism is the radical notion that women are people (Cheris Kamarae & Paula Treichler)
The reason racism is a feminist issue is easily explained by the inherent definition of feminism. Feminism is the political theory and practice to free all women: women of color, working-class women, poor women, physically challenged women, lesbians, old women –as well as white economically privileged heterosexual women. Anything less than this is not feminism, but merely female self-aggrandizement. (Barbara Smith)
There are just two pieces of dogma in my feminist tent: Society deals with gender in a way that, on balance, harms women. This is a problem that must be corrected. You’ll notice that they have nothing to do with: men, race, class, liberty, religion, teleology, biology, consumerism, violence, sex, or shoes. This is deliberate. (yami, a feminist blogger)
Feminism is not a monolith, nor is it a dogma. The only thing you have to believe in order to call yourself a feminist is that ensuring women's freedom and equality of opportunity in all spheres of life is a crucial priority. That's it. (via The Happy Feminist (a blogger), interesting post btw)
If you believe in, support, look fondly on, hope for, and/or work towards equality of the sexes, you are a feminist. (Sarah D. Bunting)
1. Advocates for the social, political, and economic equality of the sexes.
2. Believes that there is current, significant, society-wide inequality and sexism.
3. Doesn’t believe that men are the primary victims of inequality and sexism. (via Ampersand, aka Barry Deutsch)
And from what I have seen on reddit:
Feminism as a concept only exists under the assumption that in the large arena of sexual equality, women are behind men. The colloquial definition of feminism is 'equality of the sexes'.
Feminism is about equality. For everyone.
Feminism is a view that biology is not destiny and that stratification by gender is wrong and should be resisted.
Feminism is a movement encompassing a broad range of beliefs and ideas that all focus on one common goal: the end of women as second-class citizens.
Feminism is a collection of movements with varying methods for reaching the common goal of ending sexism.
I see feminism as an agent combating patriarchy and its resulting harms--not only pertaining to women, but to any gender. It's a pursuit of understanding how we're not so different across the gender, and self-actualizing in that regard.
EDIT 20-05-12: 2 more via reddit:
Maybe "The belief that there are injustices based in gender, and these injustices should be stopped"? Or, more specifically though more restrictively, "The belief that women face injustice on the basis of gender, and this should be stopped"?
"the desire to achieve equity between men and women"
EDIT 23-05-12: Well one that is slightly un-feminist
Feminism is the idea that we can make both sexes equal by focusing solely on the issues of one of them. (a youtube vlogger with the name of TJ Kincaid)
EDIT 29-06-12: More via reddit:
Feminism is recognizing society has treated men and women different, and women have been unable to participate fully because of this, and having a desire to change that fact.
Feminism is the recognition that patriarchal, gender-hierarchical structures exist in our society which serve to unfairly and unevenly oppress women as compared to men. It is also the recognition and acceptance of the responsibility to work against these social structures such that they no longer have the power to oppress women.