Life is the opposite to death. The study of life was known as biology, and, when dealing with alien life, could be further sub-categorized into xenobiology and xenology.
Defining life[]
When Lieutenant Commander Data wished to test his hypothesis if whether exocomps were considered to be artificial lifeforms, he inquired to Doctor Beverly Crusher the not-so-simple question of "what is life?".
According to Doctor Crusher's broad, mid-24th century scientific definition, "life is what enables plants and animals to consume food, derive energy from it, grow, adapt themselves to their surroundings and reproduce."
After she gave a broad definition of the term, Data inquired if "fire", which exhibited characteristics she described was considered to be alive, due to the fact that "it consumes fuel to produce energy, it grows, it creates offspring." Crusher explained that "Fire is a chemical reaction," adding, "You could use the same argument for growing crystals, but obviously we don't consider them alive."
When Data used himself as an example, he countered her definition, stating, "I do not grow. I do not reproduce. I am considered to be alive." She pointed out that in his case, he was unique, before he elaborated "I am curious as to what transpired between the moment when I was nothing more than an assemblage of parts in Doctor Soong's laboratory, and the next moment, when I became alive. What was it that endowed me with life?"
She explained that when her son had asked her that question in the past, that everything she said "sounded inadequate". It was then that she "realized that scientists and philosophers had been grappling with that question for centuries without coming to any conclusion." Her conclusion on the matter, however, was that "I think I'm saying that we struggle all our lives to answer it, but it's the struggle that's important. That's what helps us to define our place in the universe." (TNG: "The Quality of Life")
Formation of life[]
DNA, the biological foundation for many lifeforms
When Q took Jean-Luc Picard to the point of primordial Earth, at the moment when life was about to form there for the very first time, he described the event happening when "a group of amino acids are about to combine and form the first protein, the building blocks of what you call life." (TNG: "All Good Things...")
Preanimate matter was considered to be one of the building blocks of life.
Doctor Carol Marcus' Project Genesis was an artificial means of creating "life from lifelessness. It is a process whereby molecular structure is reorganized at the subatomic level into life-generating matter of equal mass." (Star Trek II: The Wrath of Khan)
When a piece of preanimate biomatter was found on an asteroid in 2377, by The Doctor, who discussed the depths of the discovery to Seven of Nine. When she explained that all she saw was an undeveloped nucleus contained in a cytoplasmic matrix, The Doctor explained that buried deep within that nucleus were primitive strands of DNA – "the beginnings of life." (VOY: "Body and Soul")