Hathyoga is the preliminary stage of rajayoga. According to the Yogasutra of Patanjali, hathyoga is a unique union of hakar and thakar. While hakar symbolises the solar elements thakar relates to lunar element.
Thus, hathyoga is the process, in which the prana is taken to the sahasrar through the Ida, pingala and sushumana nadi, by penetrating through the six chakra.