This is a really tough question to answer, I think.  It's so many different things to different people.  I like the explanation depression.com has:

This illness involves major depressive episodes alternating with high-energy periods of wildly unrealistic activity. A manic friend might, for example, call at 3 a.m. to announce in all seriousness that she's flying to Hollywood immediately to marry Robert Redford, and star in his next movie.

I dunno if I'd ever be crazy enough to want to marry Robert Redford, but hey... to each his own, right?  And just so we can have two angles, here's a definition from the National Mental Health Association:

Bipolar disorder, also known as manic depression, is a mental illness involving episodes of serious mania and depression. The person's mood swings from excessively "high" and irritable to sad and hopeless and then back again, with periods of the person's normal mood in between.

The above two definitions are similar, but I like the second one because it sort of links "high" and "irritable" together.  For me, that's a really accurate description.  I've gotten a really good handle on the way I behave when I'm "being bipolar" and I've definitely noticed that when I am manic, I tend to become more impatient and angry a lot more easily.  Of course, when I'm hypomanic, kind of like halfway between "normal" and manic, I have a blast... get lots of work done, tell lots of jokes, and I turn into this little social butterfly, etc.  Fun as it is, and refreshing sometimes, I still end up wearing myself out.

And then there are the lows....

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