A seedling is a very young plant that has just grown from a seed: a thin stem and the first tiny leaves. Gardeners start seedlings in trays, then move them into soil when they are strong enough. A tree seedling is a baby tree, not yet a sapling and nowhere near a grown tree. Farmers, garden shops, and school science classes use this word all the time; it is ordinary, not poetic. It is not the seed itself (that is what it grew from, and the seed coat may already be gone), and it is not a cutting taken from a bigger plant. Though actually, people sometimes say seedling for any tiny plant they are raising, even if they are not staring at the seed any more.