The last few remaining Rebels are not a threat.
—Supreme Commander Luke Skywalker to Emperor Palpatine
Alliance holdouts was a general term given to various insurgent groups formed after the collapse of the Alliance Remnant in 6 ABY, in the culmination of the Continuation War. The conflict began after the Galactic Empire's victory at Endor in 4 ABY, causing the Alliance to Restore the Republic to be reformed into the Alliance Remnant. In the Continuation War, which lasted two years, the declining Remnant was defeated for good, with the deaths of Mon Mothma and Gial Ackbar at the Battle of Orion IV. However, many Rebel units and ships continued resisting Imperial rule, operating independently throughout, mainly, the Outer Rim Territories. The groups had no unified government and did not recognize any one leader, and were steadily getting wiped out by the Empire (and occasionally even fighting each other). They were gone by the time of the Yuuzhan Vong invasion of the galaxy in 18 ABY.