DOHA, Qatar — Nearly 11 days after peace negotiations between the United States and the Taliban began with high hopes, it has become clear that any resolution to the 18-year war could be frustratingly slow.

One of the most prominent issues thwarting progress is a disagreement over a fundamental question: What is terrorism, and who is a terrorist?

The answer is so important because the two sides had already agreed in principle on a framework for two crucial issues: the withdrawal of American troops, and a commitment that Afghan soil would not again be used to launch terrorist attacks against the United States and its allies, as Al Qaeda did with its strikes on Sept. 11, 2001. That attack led the Americans to invade Afghanistan in an effort to hunt down Al Qaeda’s mastermind, Osama bin Laden.

The Taliban have said they would not allow Afghanistan to be used as a launching pad for international attacks. American negotiators have insisted on specifying that Afghanistan not be used by “terrorist” groups, but the Taliban have resisted, saying there was no universal definition of terrorism.