In Star Wars the Force Awakens the Resistance is called the resistance.
What are they resisting now that the empire is defunct?
This is answered in the opening crawl of The Force Awakens...
Luke Skywalker has vanished. In his absence, the sinister FIRST ORDER has risen from the ashes of the Empire and will not rest until Skywalker, the last Jedi, has been destroyed.
With the support of the REPUBLIC, General Leia Organa leads a brave RESISTANCE. She is desperate to find her brother Luke and gain his help in restoring peace and justice to the galaxy.
Yes, the Empire has gone but the forces of oppression have a new face now...The First Order..
In the decades that followed the Empire's defeat, the First Order gradually built its strength through the secret mobilization of new fleets and armies in violation of the Galactic Concordance and Republic law. Despite further acts of defiance to the treaty, the Galactic Senate did not regard the First Order as a serious threat to the Republic. However, the Senate's inaction motivated the Rebel veteran Princess Leia Organa to found the Resistance.
The war against the Empire was fought by The Rebel Alliance...not the Resistance which is a NEW organisation but obviously has similar origins and goals.