The term Mahayana refers to Mahayana doctrine, and the ideal of the bodhisattva.
In Mahayana doctrine (religious teaching), one who has advanced to near the final stage before enlightenment (nirvana) foregoes their own nirvana to first help all sentient beings reach enlightenment.
This Buddha-to-be, who has voluntarily and altruistically put off his own enlightenment for the sake of helping the world at large, is known as a bodhisattva. Since this aim is very broad in scope (saving the whole world), it is called Mahayana, with the sense of a larger, more encompassing path.
A bodhisattva uses any means possible to save others, so Mahayana Buddhism is supposedly more receptive to doctrinal innovation in the interest of propagating the Buddhist dharma.