Biologically speaking, there is no general consensus of what constitutes “life.” There is, in fact, an ongoing debate about whether viruses are “alive.”
But most biologists would agree that any organism which is “alive” must at least have these properties:
*]homeostasis (regulated internal environment, such as electrolyte concentration)
*]organization (into one or more cells, the basic building block of life)
*]metabolism (create energy to maintain internal organization (homeostasis)
*]ability to grow
*]ability to reproduce (beget new organisms)
*]adapt (respond to surroundings)
The first four are obviously present in humans at the moment of conception, whereas the last two come later. Embryos are known to adapt (respond to stimuli) well before the end of the first trimester (when it is usually easy to kill them in the United States). Reproduction does not come about until adolescence, but the genetic ability to reproduce is there from the beginning - it simply must develop to become functional.
Metabolism transforms energy to create new cells (anabolism) and organically dispose of dead cells (catabolism). The ratio of anabolism to catabolism is our “metabolic activity,” and it is a common yardstick to measure “how alive” an organism is. If more cells are dying than are being created, the “total life” of the organism is decreasing, and the organism is dying. Biologically speaking, we are “more alive” at conception than we will ever be. For the first couple-dozen cellular divisions of a healthy embryo, there is zero catabolism, meaning the mathematical ratio of anabolism to catabolism requires division by zero, which is infinite (meaning metabolic activity is infinite, which is why this measure is rarely applied at this stage - it makes biologists look like they flunked math).