Unity was a moment of spiritual sharing between two consenting parties whereby they link together on a spiritual level to become essentially one being. It was compared to experiencing a decade of exceptional sex at once, although the psychological strain of such a link can make it dangerous, suggesting that at least one participant must be trained before attempting such a bond.
Unity can also be deliberately dangerous; Zhaan killed a lover when sharing a moment of Unity with him to avenge the imprisonment of her father and the deaths of other Delvians that he had betrayed to the Peacekeepers. ("Rhapsody in Blue") During a later moment of Unity with another Priestess, her 'partner' in Unity essentially stole the decade of training that Zhaan had undergone to curb her inner darkness so that she and her followers could learn how to channel it themselves without going through the time and effort Zhaan took. Fortunately, by sharing another moment of Unity with John Crichton, Zhaan was able to take Crichton's perception of her trained self and use it to essentially re-establish the training that she had lost, allowing her to return to normal. On a later occasion, Harvey- the neural clone of Scorpius in Crichton- was able to injure Zhaan by tricking her into entering into a moment of Unity with him, initiating a bond that was able to overwhelm Zhaan. ("Die Me, Dichotomy") Zhaan was later able to use a moment of Unity to draw the injured Aeryn Sun back to life, ("Season of Death") but this bond proved fatal to Zhaan, causing her to eventually die from the loss of energy she had suffered restoring Aeyrn to life.
Those who have shared Unity retain a connection, although this can only be exploited in specific circumstances; when Moya was frozen in time while trying to escape a center halo, Zhaan was able to use the bond she shared with Crichton from their previous moment of unity to transfer her energy to him so that he could keep moving despite the current time freeze caused by the halo and perform the necessary actions to expel them back into normal space, although this transference was only possible with Stark aiding Zhaan. ("The Locket")