settle a debate about same mass compounds

[lost-scotsman]

:shock:
Me and my group are having an argument about the correct name for two different molecular species that have different composition, but the same mass. Some say that they are isobars, but I point out that that is generally only applicable for atmos, not molecules. However, I must admit I may be wrong, but I do seem to remember there being a specific word for it. I've searched my chemistry dictionary but so far got nothing. Can anybody help?

l-s