The idea came about because the research team responsible for producing the umbrella body's UK Voluntary Sector Almanac struggles to decide which organisations to include.
"There is plenty of research about the sector, but definitions vary, so it's hard to compare findings," said Oliver Reichardt, one of the almanac's authors. "It's important the sector takes the lead on this, otherwise somebody else will. The Government chose the term third sector, and lots of people are unhappy with it."
Reichardt said the NCVO was consulting widely over the issue and it could take up to two years to finalise a definition.
If it proves impossible to reach agreement across the sector, he said, the NCVO will probably drop the project altogether.
Whatever the outcome of the research, sector organisations will no doubt continue to ponder their true identity for a while yet.