c.1300, "entrance, arrival," literally "what enters," perhaps a noun use of the late Old English verb incuman "come in," from in (adv.) + cuman "to come" (see come). Meaning "money made through business or labor" (i.e., "that which 'comes in' as a product of work or business") first recorded c.1600. Income tax is from 1799, first introduced in Britain as a war tax, re-introduced 1842; authorized on a national level in U.S. in 1913.
The amount of money received during a period of time in exchange for labor or services, from the sale of goods or property, or as a profit from financial investments.