What could be simpler than to remember that income basically refers to money that "comes in," or is earned. Your income is the money you make in a given time period, for example weekly, monthly, or annually.
Your income might come from many sources — salary, investments, interest or annuities. Today income is used as a noun, but the Old English word incuman was first a verb that meant "to come in," and referred to the money earned through one's labor or business dealings. The first income tax was attempted in Britain in 1404, but was so unpopular it was ended. In the United States, the Federal Income Tax became law in 1913, and, unfortunately, is still in effect.