The term non-sentient referred to any thing or being, as was often associated with lower animals that lacked sentience; that is, at minimum, intelligence, self-awareness, and consciousness. (TOS: "The Enemy Within"; TNG: "The Measure Of A Man")
Biological lifeforms[]
According to Benjamin Sisko, "centuries ago, people on my world engaged in bloodsports, killing lower species for pleasure. A few cultures still do. But even they wouldn't consider hunting a sentient being." (DS9: "Captive Pursuit")
A line in Data's poem, Ode to Spot, defined his cat Spot as being "not sentient" because he does not comprehend. (TNG: "Schisms")
As of 2364, long range sensor scans of Delphi Ardu IV indicated that the planet had "no lifeforms, sentient or otherwise." (TNG: "The Last Outpost")
Several crewmembers and civilians of the USS Cerritos were unintentionally reduced to a single large mass of organic matter when Ensign T'Lyn transported them to the brig when stopping the ship from being taken over by T'Illups and his faction. (LD: "Twovix")
Artificial lifeforms[]
For a time in the Federation, androids and holograms were considered to be non-sentient. (TNG: "Elementary, Dear Data", "The Measure Of A Man", etc.)
When Data first applied to Starfleet Academy in the first half of the 24th century, Bruce Maddox was, according to the android, "the sole member of the committee to oppose my entrance on the grounds that I was not a sentient being." He even referred to Data by the pronoun "it". (TNG: "The Measure Of A Man")
Several years later, in 2365, Maddox made the contention that because he was not sentient, he was, according to Jean-Luc Picard's summary, the property of Starfleet, and "therefore not entitled to all the rights reserved for all lifeforms within this Federation." Picard eventually argued sufficiently in favor of Data's sentience. (TNG: "The Measure Of A Man")
Tuvok later described the hologram Marayna as "a nonsentient computer subroutine." (VOY: "Alter Ego")