It is purification by means of the path.
In ethics, tafrīd (singling out) is to indicate (ishāra) the Real (al-Ḥaqq) by the Real and through the means of messengership. In methodology (uṣūl), tafrīd is the purification of the indication (ishāra) by intending (murād) the Real and traversing (sulūk) to Him. In remedies (adwiya), tafrīd is the purification of the indication (ishāra) by love (maḥabba) and zeal (ghayra) for the Real.
In sainthood (walāya), tafrīd is the purification of the indication (ishāra) by boasting with the Real and by refraining from disclosing His secret. In realities (ḥaqāʾiq), tafrīd is the purification of the indication (ishāra) by witnessing (mushāhada) the Real and by union (ittiṣāl) with Him. (Jāmiʿ, 302)
In language, tajrīd means to remove, strip, or lift. To say "I removed the garment" means "I lifted it from myself." For a person, tajrīd (which means stripping) is to remove the garment. According to the leading Sufis, it is divided into three parts: The tajrīd of the outward (ẓāhir) is to abandon everything that distracts the limbs from Allah. The tajrīd of the inward (bāṭin) is to abandon everything that distracts the qalb (the heart) from Allah. The tajrīd that results from these two is for both the qalb and the body to belong solely to Allah and remain with Him. (Rihla, 78)
Tajrīd is to remove all that is other than Allah (mā siwā) from the qalb and the secret (sirr).
Tajrīd is to be deprived of witnessing and witnesses (the world and everything in it). There are three degrees: The first degree is for the very essence of unveiling (kashf) to be stripped from attaining certainty (yaqīn). The second degree is for the very essence of union (jamʿ) to be stripped from the degree of certainty, which is tadānī-tadallī (proximity and descent). The third degree is for ikhlas (sincerity) to be stripped from even perceiving tajrīd. (Rihla, 79)
Tajrīd is of three kinds: Some are stripped (mutajarrid) from everything other than Allah; some are stripped from the desires of their nafs (the lower self) and nature, and these are also called mutajarrid; some distinguish themselves only by serving the shaykh. (Rihla, 81)
The final stages (nihāyāt) are maʿrifa (gnosis), fanāʾ (annihilation in God), baqāʾ (subsistence in God), realization (taḥqīq), talbīs (veiling), existence (wujūd), tajrīd, tafrīd, union (jamʿ), and tawhid (the oneness of God). (Rihla, 207)
(See also: tawhid)