No single trait reliably distinguishes living organisms from non-living matter; instead, biologists rely on a cluster of properties shown together — growth, reproduction, metabolism and consciousness — of which metabolism and consciousness come closest to being truly exclusive to life, since growth and reproduction alone do not reliably separate the two.
Growth is an increase in the mass and number of cells of a body. In plants, growth is indeterminate: cells keep dividing throughout life at localised regions called meristems, so a plant never really stops growing. In most animals, growth is determinate — the body grows to an adult size and then stops, though tissues such as hair, nails and blood cells continue to be renewed. Growth in a living body is intrinsic, added from inside by cells dividing and enlarging; in unicellular organisms, an increase in cell number is itself reproduction, so the two are not separate events, whereas in most multicellular organisms they are mutually exclusive. Reproduction — the production of offspring with features similar to the parent's, by fission, budding, spore formation or sexual means — multiplies the population, but it is not an all-inclusive defining feature of life either: mules and infertile human couples are very much alive yet never reproduce.
Metabolism is the sum of all the chemical reactions occurring in an organism's body: anabolic reactions build complex molecules from simpler ones, as in photosynthesis, while catabolic reactions break complex molecules down, as in respiration. No non-living object shows metabolism, which is why it is treated as a defining feature of life — even though isolated metabolic reactions can also be carried out outside a cell, in a cell-free system with purified enzymes, and such a system is plainly not itself alive. Consciousness, biologically, is the ability to sense one's surroundings and respond to stimuli — physical, chemical or biological — shown by every organism, from a bacterium reacting to a chemical gradient, to a touch-me-not (Mimosa) leaf folding when touched, to a human perceiving and responding to the world; only in humans does it also carry the everyday sense of self-awareness.
The classic distractor pairs growth with non-living matter — a mountain, a crystal or a heap of sand also increases in mass with time. The distinguishing point is the mechanism: such non-living growth is by accretion of material on the outer surface, whereas living growth is intrinsic, arising from cell division and enlargement from within the body, so growth alone cannot separate the living from the non-living.