grass: the low green plants that carpet open ground, including the young blade of a grain crop and, once cut and dried, hay
This is the ordinary word for grass, the green stuff that covers a field. It is not a narrow botanical term. It sweeps in the wild plants and wildflowers of the open country, the tender green blade of a grain crop before its head of grain forms, and the same growth once it has been cut and dried into hay. In every case it names the low, leafy green cover of the ground, whatever stage that cover is in.
Long before the New Testament the word had meant an enclosed farmyard where animals were penned. By the first century that older sense was completely gone, and it simply meant grass. So it turns up in plain, down-to-earth settings: a crowd sitting on the grass by the lake, the first green blade in a farming parable, the dry hay stuffed into a poor roof, the vegetation scorched in a scene of judgment.
What gives the word its punch in a few famous passages is not the word itself but an image the Old Testament had already made famous. In Isaiah, all human life is grass: it springs up green, then the hot wind dries it and its flower drops, while God's word lasts forever. New Testament writers reach for grass to carry that same picture. It reassures (if God dresses the short-lived grass, he will care for you), it humbles (the rich man will fade like a meadow flower), and it lifts up God's enduring word against everything that withers.