A light note (3)
I believe the answer is:
sol
This could be a double definition.
'a light' is the first definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'note' is the second definition.
(musical note as in do-re-mi-fa-sol-la-ti)
Can you help me to learn more?
(Other definitions for sol that I've seen before include "Fifth note of scale" , "Roman counterpart of Helios" , "Note; Peruvian currency" , "Mexican lager" , "Personification of the sun" .)