a cultivated edible garden plant; a vegetable or pot-herb grown for food
This is one of the plainest words in the New Testament. It names a plant that someone has deliberately grown in a dug-over plot for eating: what we would call a vegetable, garden herb, or pot-herb, as opposed to grass (χόρτος), pasture weeds, or a woody tree (δένδρον). It almost always shows up in the plural, the way we might just say "the vegetables." There is no hidden or technical meaning to unpack, and no real disagreement among scholars about what it means.
What makes the word worth a second look is not its meaning but the work the New Testament writers make it do. Its very ordinariness gives it force. Jesus drops it into the mustard-seed parable to set up a small shock: the tiny seed grows into something "greater than all the garden plants" and puts out branches like a tree, straining the humble category the word names. In Luke he uses it to poke at religious fussiness, picturing people who carefully tithe "every garden herb" while neglecting justice and the love of God. And Paul uses it to describe believers in Rome who, out of conscience, ate only vegetables. In each case the point is carried by how everyday the word is, not by any special sense of its own.