public lecture vs speech

public lecture and speech both are nouns.

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

As nouns, speech is a hypernym of public lecture; that is, speech is a word with a broader meaning than public lecture:

Other hypernyms of public lecture include address.

public lecture (noun)	speech (noun)
a speech that is open to the public	the mental faculty or power of vocal communication
	(language) communication by word of mouth
	the act of delivering a formal spoken communication to an audience
	words making up the dialogue of a play
	a lengthy rebuke
	the exchange of spoken words
	your characteristic style or manner of expressing yourself orally
	something spoken