lambda vs alphabetic character

lambda and alphabetic character both are nouns.

Word	Noun	Adjective	Verb	Adverb
lambda	Yes	No	No	No
alphabetic character	Yes	No	No	No

As nouns, alphabetic character is a hypernym of lambda; that is, alphabetic character is a word with a broader meaning than lambda:

lambda: the 11th letter of the Greek alphabet
alphabetic character: the conventional characters of the alphabet used to represent speech

Other hypernyms of lambda include letter, letter of the alphabet.

lambda (noun)	alphabetic character (noun)
the craniometric point at the junction of the sagittal and lamboid sutures of the skull	the conventional characters of the alphabet used to represent speech
the 11th letter of the Greek alphabet