Definition: Software that is given away for free but only allows you to use it for a limited number of days, a limited number of uses or doesn't allow you to use all the features. This is their way of getting you hooked on the product and then pulling it out from under you to get you to buy it. Sometimes the ones that give you a limited version, but unlimited days or uses, are good enough for what you need them for and you can get buy without purchasing the product. If you want to use the advanced feature though you will have to buy it.
Examples:
The Web site editor I'm using is shareware. It's not free but the trial version is.