Patañjali's yogasūtra 1:8 - Viparyayaḥ

{1.8} 

विपर्ययो मिथ्याज्ञानमतद्रूपप्रतिष्ठम् 

viparyayo mithyājñānamatadrūpapratiṣṭhitam

[An] erroneous impression is based on false knowledge [in which our mind has] a conception not of its [the object's] own form

विपर्ययः= मिथ्या+ज्ञानं अतत्+रूपं प्रतिष्ठम्

विपर्ययः - contrariety, reverse, mistake; masc., noun. (Hindi speakers will recognise this as arising from the same root as विपरीत [वि+पृ 3P]) 

मिथ्या - false, untrue, incorrect; indeclinable, cognate with the Eng. myth

ज्ञानम् - knowledge, knowing; neuter noun from √jñā 9P

अतत् - not of that 

रूपम् - form, appearance, figure; neuter noun from √rūp 10P

प्रतिष्ठम् - centre, base; neuter noun from prati√sthā 1P

This vritti is often explained by commentators using Śankarācārya’s famous 'seeing rope, thinking snake' example. Basically, there is contact with an external object but the image/data received by the mind conjures a response which is not the real form of the object. Another example of this would be the story from the Rāmāyaṇa in which Daśaratha mistakes the gurgling of a water pot for an elephant.

Important to note, viparyaya only applies when there is a lack of correspondence between our mental image and the actual object. It is not concerned with erroneous images that correspond to the actual object. For instance, in a fog or the dark, you might see a blurred image of someone you know, or a building - but if it is in fact that person/building, that is not a case of viparyaya.