Hydrogen or nitrogen, say (3)
I believe the answer is:
gas
I believe this is a double definition.
'hydrogen' is the first definition.
(hydrogen is a gas)
'nitrogen say' is the second definition.
(nitrogen is a kind of gas)
(Other definitions for gas that I've seen before include "Something that is neither solid not liquid" , "Domestic fuel" , "chew the fat" , "THIRD PART OF 23" , "Chatter vapidly" .)