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mid-15c., from Latin ingressus "an advance; walking; an entry," from past participle stem of ingredi "to step into, enter" (see ingredient). The verb, sometimes said to be American English, is attested from early 14c.
in astronomy, the apparent entrance of a smaller body upon the disk of a larger one as the smaller passes between the larger and the observer-e.g., the entrance of a satellite or its shadow on the disk of a planet. The term is also applied to the Moon's entrance into the Earth's shadow at the start of a lunar eclipse and to the Sun's entrance into a zodiacal constellation.