public lecture vs lecture

public lecture and lecture both are nouns.

Word	Noun	Adjective	Verb	Adverb
public lecture	Yes	No	No	No
lecture	Yes	No	Yes	No

As nouns, public lecture and lecture are synonyms defined as:

Other synonyms of public lecture include talk.

public lecture (noun)	lecture (noun)
a speech that is open to the public	teaching by giving a discourse on some subject (typically to a class)
	a speech that is open to the public
	a lengthy rebuke

public lecture (verb)	lecture (verb)
	censure severely or angrily
	deliver a lecture or talk