series vs nexus

series and nexus both are nouns.

Word	Noun	Adjective	Verb	Adverb
series	Yes	No	No	No
nexus	Yes	No	No	No

As nouns, nexus is a hyponym of series; that is, nexus is a word with a more specific, narrower meaning than series:

series (noun)	nexus (noun)
(mathematics) the sum of a finite or infinite sequence of expressions	a connected series or group
a periodical that appears at scheduled times	the means of connection between things linked in series
a serialized set of programs
(sports) several contests played successively by the same teams
similar things placed in order or happening one after another
a group of postage stamps having a common theme or a group of coins or currency selected as a group for study or collection
(electronics) connection of components in such a manner that current flows first through one and then through the other