Define the lowest category of taxonomic hierarchy with the help of examples

The lowest category of taxonomic hierarchy is the species.

The member of a species are very closely related, they have fundamental similarities and also they can interbreed.

Two closely related species will show distinct morphological differences, e.g. Lion (Panthera leo) Leopard (P. pardus) Tiger (P. tigris).

Closely related species fall under common genus.

Panthera is the common genus.