A child born from two Goa'uld hosts. This is considered forbidden by the Goa'uld System Lords because the Goa'uld genetic memory is passed down to the offspring, making the child a threat. (Goa'uld reproduce their own kind asexually.) The Harcesis possesses all the knowledge of the Goa'uld who fathered it.
Apophis and Sha're had a little harcesis named Shifu.