Sentience
The term sentience has shared several meanings across the centuries. During the 21st century, for example, it meant the ability to feel or perceive, which may or may not have included being intelligent or self-aware. A separate concept in use at the time was the term sapient, which meant the ability to act with intelligence. So, for example, a plant could be "sentient", and a computer "sapient" with neither having to have the attributes of the other. Intelligent life was therefore called "sapient". 

That changed sometime prior to the 22nd century. At that point intelligent life was referred to as sentient. The concepts of perception and intelligence had been combined such that the word now meant an intelligent, self-aware, conscious entity deserving of rights, respect, and freedom. (; )

In the 23rd century the word "sentient" again meant what it had in centuries past: the ability to feel or perceive. Intelligent life was therefore generally called sapient rather than sentient. ()

The tide turned again in the 24th century and intelligent life was referred to as it was in the 22nd: as sentient. There was, however, no commonly understood definition of the term. Despite centuries of consideration and linguistic changes, how one determined whether a lifeform or machine was sentient, and the legal and moral implications of being sentient, were neither fully understood nor agreed upon. (; ) 

Cyberneticist Bruce Maddox gave three soft requirements during a 2365 hearing:
 Intelligence: the ability to learn, understand, and cope with new situations
Self-awareness: being conscious of one's existence and actions or aware of one's self and one's ego
 Consciousness

He was unable to truly argue, however, using these, that Data was not sentient while Jean-Luc Picard was. ()

When the  hologram was accidentally created with the gift of sentience, he initially described the sensation as though he consciously "felt like a new man."  Moriarty later elaborated, "my mind is crowded with images. Thoughts I do not understand yet cannot purge. They plague me. You and your associate look and act so oddly, yet though I have never met nor seen the like of either of you I am familiar with you both. It's very confusing. I have felt new realities at the edge of my consciousness, readying to break through." ()

Biological lifeforms 
In the 24th century, an important component of sentience in biological lifeforms was considered to be intelligence. For instance, a large brain implied sentience. () 

Another indication of sentience was a lifeform capable of sophisticated communication. (; ) 

Hard and fast definitions of sentience across biological lifeforms were not yet developed, though. For example, Bruce Maddox could not explain why a Human was sentient; he just knew that he was. () 

Despite this, some lines of demarcation were acknowledged. A house cat, for example, was considered to be not sentient. ()

According to the Hunters, Tosk, an apparently otherwise non-sentient being, "is sentient only because we have made him sentient. He has been bred for the hunt. To make it as exciting, as interesting, as he can." ()

Artificial lifeforms

Androids 
In 2365, Phillipa Louvois of the Judge Advocate General's office held a hearing in which she decided that Data was not the property of Starfleet. During the hearing the question of an android's sentience came up, but there was no formal, legal resolution on the matter. She simply acknowledged that "basic issue" – "does Data have a soul?" She answered her own question, stating that "I don't know that he has. I don't know that I have. But I have got to give him the freedom to explore that question himself." () Despite a lack of official acknowledgment, Data thought himself to be sentient, and many others agreed. (, et al.) Later, during the creation of his daughter Lal, Data noted that with the successful transfer of her heuristic associative pathways, she "will now begin to process information on logic, aesthetics, metaphysics, and epistemology," and with that, "you are truly becoming sentient." () As of 2371, Data was considered the only sentient artificial lifeform in Federation society. () 

By the end of the 24th century, researchers in cybernetics, such as Agnes Jurati, readily acknowledged the potential sentience of advanced androids such as Data, despite their inability to replicate Dr. Soong's work. ()

From time to time other non-android lifeforms or artificial intelligences have been considered sentient as well. In 2375 the  crew determined that a weapon was so sophisticated that it was actually sentient. () An Automated Personnel Unit designated 3947 was also considered to be sentient when reactivated by B'Elanna Torres in 2372. ()

Holograms 
Holograms have also been referred to as both artificial lifeforms and non-sentient. This was not to say that a few examples of sentient holograms did not exist. 

One such sentient hologram was created on the  in 2365, when Lieutenant commander Geordi La Forge requested that the holodeck create an opponent worthy of Data in a Sherlock Holmes-style mystery. The ship's computer produced a sentient version of James Moriarty, Holmes' nemesis. () 

The Federation later encountered a group of Yaderan refugees in 2370 only to discover the entire colony but one man was made up of sentient holograms. ()

An early legal case in the arena of holograms and sentience came about in 2378. The Doctor attempted to publish a holonovel entitled Photons Be Free, but it was appropriated and released without his permission by Broht & Forrester. The legal issue revolved around whether The Doctor was an "artist" within the meaning of the laws that granted rights to control the dissemination of intellectual property. The ruling was narrow in that the definition of artist in that single law was extended to a hologram, but it was an important step on the path toward granting full legal status to a hologram as a sentient entity. ()

Societies other than the Federation have also wrestled with these concepts. In 3074, for example, the Kyrian fully recognized artificial lifeforms – including holograms – as sentient. ()

Aspects of consciousness
 Free will
 Psychology and psychiatry 
 Ego
 Superego
 Id
 Subconscious
 Metaconscious
 Quantum consciousness

See also 
 Sentient cave
 Sentient tar
 Wisdom

Appendices

Background information 
The English word "sapient" means "having intelligence". In that respect  was much more correct in its use of language than any subsequent series. The words sentient and sapient are commonly misunderstood and misused by people, though, and the (incorrect) meaning of sentient as used in  and beyond has been firmly cemented in Trek to mean a combination of sentience and sapience.

External links 
 
 
 
Philosophy