The modified stem of Opuntia is
phyllode
phylloclade
cladode
stamroinode
B.
phylloclade
When the stem is flattened and function as leaf, it is called phylloclade or cladophyll.
Phyllode is a flattened petiole or leaf rachis that resembles and functions as a leaf and may or may not be combined with an actual lamina.
Cladode is a flattened origin arising from the stem of a plant.
Staminode is a sterile or abortive stamen, frequently resembling a stamen without its anther.