harvest: the reaping and bringing in of a ripe crop, whether the crop itself, the act of cutting it, or the season it falls in.
This is the ordinary word for the grain harvest: the ripe crop, the cutting of it, and the time of year it happens, all carried by one word. In the Gospels, though, it is almost never about actual grain. Jesus takes the familiar sight of a field ready to be cut and makes it stand for people.
In one direction the harvest is the gathering of people to God. The field is full of men and women ready to be brought in, and the shortage is not of ripe grain but of workers to gather it (Matthew 9:37-38; John 4:35). Here the mood is opportunity and urgency, an errand of rescue, not a threat.
In the other direction the harvest is the end of the age, when God gathers in what has grown and separates the good from the bad (Matthew 13:39; Mark 4:29; Revelation 14:15). The same picture now carries a reckoning: the wheat is stored, the weeds are burned. The two uses are not sealed off from each other, and a single scene can lean toward the ingathering while keeping the shadow of the final sorting.
The verb behind these scenes is θερίζω, the act of reaping, and the one who does it is the θεριστής, the reaper. The Old Testament harvest word קָצִיר stands behind the imagery, the ripe field and the sickle that later prophets and Revelation reach for.