Vikalpa is a Sanskrit philosophical term used in Hinduism and Buddhism, meaning erroneous conceptualizations ("concepts, judgements, views, and opinions") which are coloured by emotions and desires.
[1] Nirvikalpa is the absence, or 'seeing-through', of these erroneous mental constructions, as aimed for in yoga and meditation, in which both the calming of the mind (samatha, samadhi) and insight into the workings of the mind (prajna, bodhi, vipassana) are sought after.
Commentators take vikalpa to be metaphorical or figurative language that conveys meaning but lacks a basis in physical reality.
Similarly, expressions like "the sun rises and sets" or "time flies" are vikalpas; they do not describe literal truths but are universally understood.
This sets it apart as a distinct vritti (mental modification) in the Yoga Sutras.