(covers information from several alternate timelines)
Saliva or spit was a watery substance located in the mouths of animals and humanoids, secreted by the salivary glands.
In March 2153, Doctor Phlox located a marsupial on a class L planet whose saliva was a natural anticoagulant. (ENT: "Bounty")
Months later, upon being presented with Kessick's severed finger, Phlox commented that a saliva sample would have been sufficient to confirm that Kessick was a Xindi-Primate. (ENT: "The Xindi")
In the mirror universe in 2155, Phlox found traces of reptilian saliva in Kelby's wounds, after the ensign had been killed by Slar. (ENT: "In a Mirror, Darkly, Part II")
In 2370, Data discovered Klingon DNA in the form of saliva in Deanna Troi's wound inflicted by a de-evolved Worf. (TNG: "Genesis")
In 2374, Dejaren described his contempt of organic beings' eating practices as, "Secreting saliva to force food down your esophagus into a pit of digestive acids." (VOY: "Revulsion")