The prevailing definition of the raga is रंजयति इति राग: (what pleases the mind is a raga). Personally I found this definition rather too general and not specific. Hence I developed the definition based on the structure of the raga rather than generalizing – as follows.
a. Raga (concept)
The musical scale contains 7 + 5 notes (7 shuddha and 4 komal & 1 tivra).
From these 12 notes selected notes, minimum 5 are used to build up a melodic structure called a raga. It is believed that there are about 300 ragas. However, only around 50 are heard in performances. The lesser known and difficult to perform ragas are called achhop and anvat respectively.
b. Every raga has melodic centres, two of which include – one of principal importance and the other a little less important, called vadi and samvadi respectively. Notes which support vadi and samvadi notes are called anuvadi notes. Notes that are excluded from the raga structure are called vivadi notes.
c. Ragas with melodic movements centred in the first half of the scale (Sa to Ma or Pa) are called purvang ragas and the second half of the scale (Ma or Pa to upper octave Sa) are called uttarang ragas.
d. According to tradition, every raga has a definite time period for performance, during the time cycle of 24 hours. A large section of the music community accepts this concept. Some believe that the concept is scientifically based on an intimate relationship between sound and light.
e. Tradition also has it, that specific ragas are performed during specific seasons such as spring, monsoon, etc. e.g. Basant, Bahar, etc. in spring time and Miyan Malhar, Desh etc. in the monsoons.
f. It is generally accepted that there are 9 basic emotions (rasas), some of which form the essential ingredients of any performance in the field of music, dance and theater. Every raga therefore has a basic emotion (rasa) as an undercurrent of its exposition. It is however incorrect to believe that a complete raga exposition from alap to jhala is based on only one rasa. Allied rasas do play significant role in raga development, depending upon the area of the melodic development, or the speed and the tempo at which the raga is performed.
[Note: A melodic structure cannot be called a raga unless it is accepted / adapted by the music community at large.]