c. 1300, porche, "covered entrance; roofed structure, usually open on the front and sides, before an entrance to a building," from Old French porche "porch, vestibule," from Latin porticus "covered gallery, covered walk between columns, arcade, portico, porch," from porta "city gate, gate; door, entrance," from PIE root *per- (2) "to lead, pass over."
The Latin word was borrowed directly into Old English as portic. Especially (late 14c.) "a covered walk or colonnade on the front or side of a building." In U.S., used by 1832 for what the British call a veranda.
1823, "gateway to an Egyptian temple," from Greek pylon "gateway," from pylē "gate, wing of a pair of double gates; an entrance, entrance into a country; mountain pass; narrow strait of water," a word of unknown etymology, perhaps a foreign technical term. The usual word for "door" in PIE in Greek took the form thyra (see thyroid). Meaning "tower for guiding aviators" (1909) led to that of "steel tower for high-tension wires" (1923).
Meaning "large entrance hall in a public building" is from 1590s; in reference to the House of Commons from 1630s. Political sense of "those who seek to influence legislation" is attested by 1790s in American English, in reference to the custom of influence-seekers gathering in the large entrance-halls outside legislative chambers.