Act.  Salvation.  Emancipation.  Liberation from this Sansaar (world).  Mukti.  Moksh.  It is the fifth state in which modification is extinct, and when it remains alone with its own nature. The word "Kaivalya" is derived from the word "Keval" meaning "only". Kaivalya gives one absolute bliss since one can enjoy Nir-Gun Brahman ie pure Consciousness without the limitations of the Dehaatm Buddhi which is the real bondage.

A-Dwait View
There are two terms used for Kaivalya - Self-realization (Aatm Saakshaatkaar) and God-realization (Bhagavat Praapti). But there is a subtle difference between these two terms. Ved highlight importance of the former while Puraan like Bhaagvat Puraan, Vishnu Puraan etc. highlight the importance of reaching the feet of Naaraayan or Hari. In the broader sense both expressions mean the same end. In the Geetaa, Krishn advises Arjun a number of times to worship Him alone (manmanaa bhava, madbhaktah, mayyeva mana aadhatsva, madyaajee mam namaskuru, maamaiva eshyasi, maam ekam saranam vraja etc etc) and if Arjun worships Him with single-pointed attention (Ananya Bhakti), he would come to Him alone.

A closer reading of the Geetaa reveals that Krishn is not talking only as Krishn, son of Vasudev or as Shreeman Naaraayan, but as the Supreme Being (Paramaatmaa), who has no form (Nir-Aakaar), no attributes (Nir-Gun) etc etc. That Supreme Being is the Self (Aatmaa). Hence the expressions Self-realization and God-realization (Eeshwar Praapti or Bhagavat Praapti) refer to the same ultimate goal (Param Dhaam, Param Padm etc) of beings. The Purush Sookt of Rig Ved says - "Purush evedagum sarvam.... ....meaning Purush alone is the entire world, both past and future; he is also the Lord of immortality. This great Purush, brilliant as the Sun, who is beyond all darkness (Tamasah Prastaat) I know Him in my heart. Whoever knows the Purush thus attains immortality in this very birth. I know of no other way to salvation (Naanyah Pantaa vidyate ayanaaya).

Take a look into a couple of the selected verses from Bhaagvat Puraan to understand this terminology:
--Verse 3.11.2 - Here the word Kaivalyam is used in the sense of 'Oneness'. Atoms are the ultimate state of the manifest universe. When they stay in their own forms without forming different bodies, they are called unlimited oneness (Kaivalyam). They are certainly different bodies in physical forms , but the atoms themselves form the complete manifestation.
--Verse 3.27.19 - Here Kaivalyam has been used to mean 'freedom'. The verse says, although the soul is the passive performer of all activities, how can there be freedom (Kaivalyam) for the soul as longs as Gun (material nature) acts on it and binds it.
--Verse 4.20.10 - In this Shlok, the term Kaivalyam has been used to mean freedom from material contamination. It says, when the heart is cleansed of all material contamination, the devotee's mind becomes broader and he can see things equally. At this stage of life there is peace, and one is situated equally with Me .
--Verse 10.51.20 and 11.30.34 - Kaivalyam is used here to mean liberation. In the former verse it is said that only the Lord Vishnu can bestow it, whereas in the latter one the Lord says, because, "My devotees possess saintly behavior and deep intelligence, they completely dedicate themselves to Me and do not desire anything besides Me. Indeed even if I offer them liberation from birth and death, they do not accept it."

There is one Upanishad also named after "Kaivalya". (See the List of Upanishad)

Vaishnav View
Muc = to be free; Mukt = one who has been liberated from cycle of birth and rebirth.
Jeevan-mukt = living being who is freed. Corporeal liberation. Shiv sect believes in this, but Vaishnavites don't accept this.
Videh Mukti = liberation after death. Incorporeal liberation. This is Vaishnava Tradition.
Mumukshu = One Longing for liberation from bondage.
Kaivalya = Self-realization; A state in which the soul is freed from corporeal burden in A-Dwait tradition.
It is NOT God-realization, because you are not at the Feet of Shiv or Vishnu.
Moksh or Mukti = liberation from fetters, bondage, A-Vidyaa, metempsychosis.

HTML clipboard Gyaan Yog is not generally recommended for salvation in Vaishnavism; it leads to enlightenment and bliss of the soul and freedom from metempsychosis. All this amounts to Kaivalya and not God-realization. Kaivalya is Aatm-Anubhav (Self experience) and NOT Bhagavat-Saakshaatkaar. Aatm Anubhav is the realization of the knowledge of one's own soul and not the realization of God . It is knowing oneself and NOT the God. Kaivalya guarantees complete release from mundane existence and thus lets the individual soul enjoy its true nature.

Kaivalya is not enjoying the Universal Soul (Bhagavaan). The true Moksh is to serve God, without any interruption, in Vaikunth or Parmapad. Real Moksh is simply not a state of negation (which is Kaivalya), absence of bondage; rather it has a positive aspect to it in that one gets close to Bhagavaan, gets touched by Bhagavaan, and perform interminable eternal service to Bhagavaan. In modern parlance, the Moksh Purush has earned his tenure and no one can fire him from Vaikunth.