a sickle: the short curved blade a harvester swings by hand to cut a crop that has ripened
A sickle is a short curved blade set in a wooden handle, the basic hand tool of an ancient harvest. The reaper gathered a fistful of standing stalks with one hand and swept the blade through them with the other, moving across the field a handful at a time. A similar hooked blade, usually smaller, served the vineyard, where it cut ripe clusters free of the vine.
Every use in the New Testament is a picture rather than a description of farm work. Jesus ends a short parable with a farmer who puts in the sickle the moment the grain is ready, because the crop, not the farmer, decides when the time has come. Seven of the eight uses fall in a single scene in Revelation, where figures with sharp sickles reap the earth: one cuts the grain, another gathers the grape clusters and throws them into the winepress.
Both scenes reach back to Joel 3:13, "put in the sickle, for the harvest is ripe" (מַגָּל), and that line supplies the logic of the image. A sickle is not raised early. It waits for ripeness, and once the crop is ready the waiting is over. That is what sets it apart from the axe (ἀξίνη) that John the Baptist saw laid at the root of the trees: the axe cuts down what has failed to produce, while the sickle takes in what has finished growing. The word for the harvest itself (θερισμός) names the crop and the season; this word names the blade that does the cutting.