A rule may refer to any of the following:
1. In general, a rule refers to a set list defining what is allowed. For example, a user may set up a rule on a network that prevents users from accessing portions of a network.
2. When referring to e-mail, a rule is a step to follow when e-mail is received. For example, if an e-mail had a keyword such as "viagra" the e-mail would be automatically deleted or marked as spam.
3. When referring to a word processor, a rule is a solid line on a page that helps visually separate columns of text.