Vikalpa is a Sanskrit term referring to one of the five types of thoughts, or activities and motions, within the mind. These are called vritti in Sanskrit and the great sage, Patanjali, references them in his definition of yoga: yoga citta vritti nirodha (“yoga is the silencing of the disturbances of the mind”).
Vikalpa is imagination, fantasy or illusion. It can mean thinking of situations from the past and imagining different results, daydreaming, thinking of things that do not exist or imagining future situations and events. It is an idea that does not exist in reality.