to go in, to enter a place
At its plainest this word means to move from outside to inside: a person walks into a house, a town, a room, the temple, a boat, or a tomb. It is one of the most common motion words in the Gospels and Acts, and most of the time it is simply reporting where someone went.
From this ordinary picture of going inside, the New Testament stretches the word in two directions that go beyond walking through a door. One is moral and spiritual: to enter the kingdom of God, to enter life, or to enter God's rest, where entering names crossing over into a new state of belonging rather than stepping into a building. The other is the picture of something getting inside a person, such as an unclean spirit or even Satan, or food going into the mouth. These uses all keep the same core idea of crossing from outside to inside, which is why they remain one word; the senses below mark where that crossing is physical, where it is a change of standing before God, and where it is one thing entering another.