the seed-bearing head of a grain plant: the ear of wheat or barley that stands at the top of the stalk
This is the ordinary Greek word for the tuft of kernels at the top of a cereal stalk, the part a person could pull off and rub between the palms to free the grain inside. It names a plain piece of farm life, the ear of wheat (or barley) that a Galilean field was full of from late spring into early summer.
Three of the Gospels use it for the same scene: the disciples walk through a field on the Sabbath and pluck heads of grain to eat, and they are challenged for it. Pulling ears by hand was a customary right long granted to a hungry traveler in Israel's law, so the plucking itself is not the problem, only the day. The Gospel writers reach for the very word the Greek Old Testament had used for that custom, so the moment quietly leans on Scripture against the objection.
The word also marks a stage of growth. In Jesus's parable of the seed that grows on its own, the plant comes up in three steps: first the green blade (χόρτος), then the bare head, then the head packed with ripe grain (σῖτος). Here the word is the middle stage, the head that has pushed up out of the stalk but is not yet full.