In botany, tepal is defined as that part of the flower where sepals and petals are undifferentiated.
In this case it is not possible to distinguish petals or sepals in the flower. They are in practice that part of the flower that replace the sepals and the petals and form the perigonium (the external part of the flower).
In general in the flower of the Angiosperms the name of tepals is used to indicate the conformations of the flowers in which the antofilli are not distinguishable in sepals and petals.
A typical case is that of the lily flower in which it is not possible to identify, at least at first glance, the presence of sepals, since the three antofillis that form the calyx and the three that form the corolla have the same appearance as petals.
The set of tepals takes, as mentioned, the name of perigonium, a type of perianth in which, in fact, sepals and petals are not distinct.
The existence of tepals is a fairly common morphological characteristic in Monocotyledons (eg tulips, lilies, etc.) A further distinction of tepals is the chromatic one. The tepals are called petaloid in the case in which they are colored and thus resemble the petals and sepaloid in the case in which they are greenish and resemble sepals.