Cactus and Prickly pear cactus

Cactus noun - Any succulent plant of the family Cactaceae native chiefly to arid regions of the New World and usually having spines.

Prickly pear cactus and cactus are semantically related. In some cases you can use "Prickly pear cactus" instead a noun "Cactus".

Prickly pear cactus noun - Cacti having spiny flat joints and oval fruit that is edible in some species; often used as food for stock.

Cactus and prickly pear cactus are semantically related. You can use "Cactus" instead a noun phrase "Prickly pear cactus".