The term “Sapaat (सपाट)” means something that is straight, flat, smooth, linear, not coiled, not twisted, non-curving, or following a straight line.
The term “Vakra (वक्र)” means something that is crooked, zigzag, curved, non-linear, not straight, not smooth, coiled, twisted, or not following a straight line. Vakra is the opposite of Sapaat.
Sapaat Chalan:

Vakra Chalan:

A Raaga having Vakra chalan in its Aarohan or Avrohan or both, is known as a Vakra Raaga.
Another classification of Vakra Raaga is by the number of Vakra swaras that it contains. For example:
Ekaswara vakra (having one Vakra swara; eka = one)
Dviswara vakra (having two Vakra swara; dvi/dwi = two)
Triswara vakra (having three Vakra swara; tri = three)
Chatusswara vakra (having four Vakra swara; chatus = four)
Generally, a Raaga does not contain more than four Vakra swaras.
Few examples of Raagas having Vakra Chalan:
Adana: S R m P d n S’, S’ d n P, m P g m R S
Basant: S G M d N S’, S’ N d P M G M N d M G r S
Deshkar: S R G P D S’, S’ D P D G P G R S
Desi: S R m P n S’, S’ P D m P R g S R, ‘n S
Dev-Gandhaar/Dwi-Gandhaar : S R G m P m g R G m P d n S’, S’ n d P m P g R G m g R S
Gunkali: S r m P d S’, S’ d P m r S, ‘d S
Gurjari-Todi: S r g M d N S’, S’ N d M d M g r g r S, d, ‘N S
Kamod: S R P, G m R S, m R P, D N S’, S’ N D P, M P D P, G m P, G m R S
Kausi-Kanada: ‘n S g m d n S’, S’ n d P m g R g m R S
Khambavati: S R m P D S’, S’ n D P D, m P, G m S, R, ‘n, ‘D S
Nayaki-Kanada: S R (m) g m P n P S’, S’ n P n P m P (m) g, g m R S
Puria-Dhanashri: ‘N r S, ‘N r G M P, M d N S’, S’ N d P M G M r G r S
Shuddha-Sarang: ‘N S R M P N S’, S’ N D P M P M m R S, ‘N S