Plural of harvester: people who gather crops, or machines designed to cut and collect crops.
From harvest plus the agent suffix -er (one who does something) and plural -s. The machine sense emerged in the 19th century with industrial agricultural advancement.
The Combine harvester (called that because it combines reaping, threshing, and winnowing) can harvest a football field of wheat in minutes—something that would take dozens of people a full day in the 1800s.
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