A highly simplified definition from this paper. You might want to read the rest of the paper for more informatiion:

This study, based on the 1976 Center for Political Studies Election Study, suggests that at least four different belief systems exist among the American public. We treat the dimension of government intervention in economic affairs as distinct from the dimension of expansion of individual liberties.



The liberal supports government economic intervention and the expansion of individual liberties;

the conservative opposes both.

The libertarian supports expanded individual liberties but opposes economic intervention.

The populist supports economic intervention but opposes expansion of individual liberties.

Some 72.4 percent of the sample can be classified as holding consistent or nearly consistent political beliefs using these categories.



By including libertarian and populist, this set of definitions is actually more precise than most. In real life, even more subcategories can be split off, and few people are completely one or the other, but you can see that by the standards of their survey, almost 3/4 of the people surveyed fit one of these categories closely.

I have a serious problem with this in that few conservatives I know would say they were against expanding individual liberty. A quick look at the survey seems to equate "individual liberties" with legal marijuana, which is not what I would put at the top of the list, but there it is.

Another answer, also oversimplified, is that liberals tend to be collectivist, seek centralized government solutions to problems, and tend to believe in the individual as subservient to society. Conservatives tend to be individualist, although in moral areas they can become collectivist. Libertarians and objectivists are pretty much individualist all the way down the road.

I tend to take things case-by-case. "Does this particular thing make me more free or less free?" Intelligent liberals or conservatives do the same thing, which is why you will find some people on this board often taking positions that are not down the center of the party line.