Definition - What does Green Card mean?
Justipedia explains Green Card
Prior to 1977, USCIS Form I-551 was known as the I-151 Form or the Alien Registration Receipt Form. The I-151 form was created after World War II; from 1946 to 1964, it was printed on green paper. This is how it became known as the green card. After 1964, the government stopped printing the I-151 Form on green paper, but the name stuck. In May 2010, USCIS Form I-551 reverted back to its original green color. Under U.S. law, permanent residents are required to carry their green card with them at all times. If they fail to do so, they can face a fine or imprisonment.