Just like these (savitarka and nirvitarka), savichar and nirvichar correspond to the levels of realisation of subtle subjects.

The subjects may be subtle or gross (physical). While in case of gross subjects, the experience gets mixed with logic and reason and past experiences, in case of subtle experiences it gets mixed with the thought pattern of the practitioner, such an experience coloured with the thoughts of practitioner is called savichar. When a sadhak is able to still his thoughts, the shunya-like form of the subtle subject reveals itself in its true essence, this is called nirvichar. Indications of stilling of your thoughts are - your eyeballs are still they don't move and you are not attached to anything which is physical, emotions and relations included.