That's the name yogis gave to the complex state of a kind of disillusionment with mundane life that leads to a wholehearted longing for a deeper investigation into the inner workings of the mind and the self.
Patanjali uses the term samveganam in the Yoga-Sutra 1.21: "For those who seek liberation wholeheartedly, realization is near." In Sanskrit: tivra-samveganam asannah. Chip Hartranft translates the word samveganam to "with vehemence".
Patanjali uses the word to indicate a vehement determination to find a way out of suffering (dukkha) which we can do through the practice of yoga (YS 1.22).
What samvega brings with it is a hunger for internal quiet and slowing down (nirodha) - as a reaction to the exhaustion from living a "mundane" life but also to see more clearly to understand our true nature; to still the patterning of consciousness (YS 1.2).