Stealth technology has been introduced in conjunction with AWACS vessels during the Main FreeSpace 2 Campaign. The only canon stealth spacecraft are:
A stealth spacecraft is all but invisible to enemy radars and can only be detected via visual contact. This kind of spacecraft are used in reconnaissance missions because their weaponry, armor, and shielding are too poor to survive prolonged battle.
In Silent Threat, the Loki-class recon fighter was also referred to as "stealth", although its capabilities were greatly inferior to those of more advanced craft like the Pegasus or the Ptah. Some references to the Zeus's supposed stealth capabilities can also be found in ST.