pillar of Islam vs hajj

pillar of Islam and hajj both are nouns.

Word	Noun	Adjective	Verb	Adverb
pillar of Islam	Yes	No	No	No
hajj	Yes	No	No	No

As nouns, hajj is a hyponym of pillar of Islam; that is, hajj is a word with a more specific, narrower meaning than pillar of Islam:

pillar of Islam: (Islam) one of the five religious obligations accepted by all Muslims
hajj: the fifth pillar of Islam is a pilgrimage to Mecca during the month of Dhu al-Hijja; at least once in a lifetime a Muslim is expected to make a religious journey to Mecca and the Kaaba

Other hyponyms of pillar of Islam include shahadah, salaah, salaat, salah, salat, sawm, zakat, hadj, haj.

pillar of Islam (noun)	hajj (noun)
(Islam) one of the five religious obligations accepted by all Muslims	the fifth pillar of Islam is a pilgrimage to Mecca during the month of Dhu al-Hijja; at least once in a lifetime a Muslim is expected to make a religious journey to Mecca and the Kaaba