What is the difference between a molecule and a crystal

Molecules of ionic compounds are presumably not often encountered in practice, because ionic compounds generally have high boiling points. But they do exist. Gas phase NaCl is shown here to be a mixture of NaCl monomers and dimers.

https://www.era.lib.ed.ac.uk/bitstream/handle/1842/2601/McCaffrey%20PD%20thesis%2007.pdf;jsessionid=52552542FBA223765122707815CD67F0?sequence=2