Lord Vishnu has many types of avataras. In fact, His avatars are innumerable:
O brāhmaṇas, the incarnations of the Lord are innumerable, like
rivulets flowing from inexhaustible sources of water. [SB - 1.3.26]
But these ten are only the prominent ones; especially because they take place on this material plane, our earth.
avatarati it avatara - so those are called avatarasbecause they descend to earth and take up material bodies of transcendental nature. Where as many other avatars take place on other planes of existences, just like the Mohini one. Hence, Mohini is not in the list of the ten avatars.
For a 'person'(don't get offended) to be called Avatara, he/she has to be born on the our Earth in material form experience Earth-ness but without loosing any divine power.
The word, Avatar itself mean to descend as saviour as अव- (ava-, “down”) + तार (tāra, “savior”).Wikitionary
By definition an incarnation is different. It requires the birth of God in physical form, through the natural process and his existence upon earth in physical form undergoing the same experiences as living beings. It is not necessary that the physical form needs to be that of human always. It can be a human, animal or semi human or even mythical form. The incarnation may exist for a brief period of time or for a life time. The incarnation may have all the powers and awareness of God in His absolute aspect or only a necessary portion of it.
Bhagavata Purana do list Mohini as an Avatar of Vishnu in the 26 Avatars, but also comments on it being infinite. It was Garuda Purana which counts the Avatars to be 10.
Source Srimad Bhagavatham
vaivasvatha manvantra have 10 avatara of vishnu, infact Kurma avatara resulted in 3 avatars (kurma, danvantri and mohini) they took first one kurma..
Similarly vamana avatra resulted in 2 avatara (Vamana and tiruvikrama)
infact dat atreya, vedyvayasa are also related to current manvantra
Why this 10 regarded highly.. because its related to nithya upasane.. :-) discover yourself
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