As Webster1913 states, pyriform literally means pear-shaped. However, many of us may think of a pear shape as something like a sphere with a smaller, 'parabolic dome' protruding from one side. In practice, pyriform is usually used for something more akin to a teardrop shape, although the exact usage varies.
In anatomy, structures such as the pyriform fossa and the pyriform sinus do generally look like elongated teardrops or vesica piscis, but with some variation, as is common in soft anatomical structures. In these cases the term refers to the two-dimensional profile of a three-dimensional structure, so the angle that it is viewed from strongly affects the perceived shape.
In oology, pyriform refers to a pointed egg shape: very much like a teardrop, but instead of the thin end tapering to a point it closes off with a tight curve. These eggs are common in many birds, including many songbirds, penguins, and murres.
The usual chicken-egg shape is particularly well designed for easy rotating in the nest, which keeps the chick healthy. A pyriform egg, particularly an elongated pyriform egg, will roll less easily, sitting neatly alongside its nestmates; in larger clutches, these
eggs pack neatly and compactly into a star-shaped cluster, with all the points facing inwards. Cliff-dwelling birds such as the murres benefit from the fact that this shape tends to roll in tight curves, reducing the likelihood that it will roll off of the rocky edges of seaside cliffs. Penguins prefer this shape as well, as it is ideal for sitting the eggs comfortably on top of the parent's feet.