plumbago vs pencil lead

plumbago and pencil lead both are nouns.

Word	Noun	Adjective	Verb	Adverb
plumbago	Yes	No	No	No
pencil lead	Yes	No	No	No

As nouns, pencil lead is a hyponym of plumbago; that is, pencil lead is a word with a more specific, narrower meaning than plumbago:

plumbago: used as a lubricant and as a moderator in nuclear reactors
pencil lead: mixture of graphite with clay in different degrees of hardness; the marking substance in a pencil

Other hyponyms of plumbago include lead, pencil.

plumbago (noun)	pencil lead (noun)
any plumbaginaceous plant of the genus Plumbago	mixture of graphite with clay in different degrees of hardness; the marking substance in a pencil
used as a lubricant and as a moderator in nuclear reactors