There are many routes in introducing drugs into the body. Hypodermic injection per se, is introduction of drugs into a part, as into subcutaneous tissues, the vascular tree, or an organ. the prefix hypo means "under" while the latin word derma refers to the "skin". Transdermal administration on the other hand, is the route wherein active ingredients are delivered through dermis or skin only. One example is the transdermal patches.
Thus, hypodermic needle administration is not called transdermal, it is rather called subcutaneous administration.