Lao-zi vs Lao-tzu

Lao-zi and Lao-tzu both are nouns.

Word	Noun	Adjective	Verb	Adverb
Lao-zi	Yes	No	No	No
Lao-tzu	Yes	No	No	No

As nouns, Lao-zi and Lao-tzu are synonyms defined as:

Lao-zi and Lao-tzu: Chinese philosopher regarded as the founder of Taoism (6th century BC)

Other synonyms of Lao-zi include Lao-tse, Laozi.

Lao-zi (noun)	Lao-tzu (noun)
Chinese philosopher regarded as the founder of Taoism (6th century BC)	Chinese philosopher regarded as the founder of Taoism (6th century BC)