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 James Moriarty 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."