The term stealth stands for stealth and is known from the US military's stealth bombers, which cannot be detected by radars. This is also where the name for stealth viruses comes from.
Stealth viruses are viruses that camouflage themselves by modifying system programs. When the user accesses an infected file, he cannot detect the stealth virus because it has removed itself from the file. The same applies to antivirus programs. After the antivirus program finishes, the file gets infected by the stealth virus again.
Various malware like Snoopware can run in stealth mode in smartphones and monitor voice communication, short messages and voice mails.