Wow, Webster 1913, could you BE any more vague?

More appropriately defined, the archegonium (pl. archegonia) is the multicellular reproductive organ found in bryophytes and some seedless vascular plants which produces a single egg.

Archegonia are found in the female reproductive organ of the moss or liverwort, and are suspended above the leafy structures by a stalk called the archegoniophore. In most species, the reproductive structure is shaped like a mushroom, with the stalk supporting a down-turned cup structure. This cup is called the archegonial head, and under the cap are found the archegonia themselves. They are tubular in shape, and have a single cell wall surrounding the egg. Spores from the antheridium of the same plant (if it is a self-fertilizing species) or a neighbouring plant are captured by the archegonial head and are transfered to the archegonium, where they are moved along the tube towards the egg for fertilization. After fertilization, the cells of the archegonium are re-absorbed in order to provide food for the developing embryo.

While this structure is most often found in the bryophytes, it can also be seen in Equisetum sp. and in Gingko biloba, a gymnosperm.