Other forms: entered, entering, enters
To enter is to go into a place. Enter the room, enter the building, do not enter. It is more formal than go in, which is what people actually say.
It takes no preposition — enter the room, never enter into the room — and that is the commonest error with the word. Enter into does exist and means something else: enter into an agreement, enter into a discussion. Two other everyday senses: to enter a competition is to take part, and to enter data is to type it in. Note the Enter key, and that entry is the noun for most senses while entrance is the way in.
1verbenterTo go into a place.
“Nobody may enter the building after six.”
synonyms: go in, come in
2verbputTo put information into a form or computer.
“Enter your postcode and press return.”
3verbtakeTo take part in a competition.
“Four hundred people entered and six finished.”
4verbbeginTo begin a period or a state.
“The talks have entered a difficult phase.”
Too few real examples of this word to estimate which sense is commonest.
enter is a way of going in.
Latin intrare comes from intra, within — the same root as interior, internal and intimate. To enter is to get within, and the word says nothing about doors.
Proto-Indo-European
*terh₂-
Latin
intrō — enter
Old French
entrer
Middle English
entren
English, today
enter
Built from
From Middle English entren, from Old French entrer, from Latin intrō (“enter”), from intrā (“inside”). Has been spelled as "enter" for several centuries even in the United Kingdom, although British English and the English of many Commonwealth Countries (e.g. Australia, Canada) retain the "re" ending for many words such as centre, fibre, spectre, theatre, calibre, sombre, lustre, and litre.
Etymology from Wiktionary, CC BY-SA