A solar day in the Farscape Universe is a unit of time which is approximately equal to 24 hours, or one earth day.
The terminology "solar day" is likely a bastardization of human terms introduced for viewer convenience. The word "solar" derives from the Latin "Sol," which refers to Earth's sun. It seems unrealistic that anyone in the Farscape Universe would acknowledge the universality of the relatively random measurement of 24 hours, or would have any interest in the length of a day on Earth. (That said, it is common in science fiction for the term "solar" to be used in relation to any local star when discussing things such as solar flares or eclipses.) In terms of the Farscape universe, remembering that all Crichton (and viewers) hear comes from the translator microbes, it may simply mean the terminology used to describe the concept of "day" translates into a recognizable word for John (as does cycle), whereas other terms such as arn and most swear words are not converted into English equivalents.