With two interacting bodies, interacting via the r-squared forces of gravity or electricity, and obeying classical mechanics, the equation of motion can be written down in closed form.
With three interacting bodies, the equation of motion can’t be written down in closed form.
So the three-body problem can’t be solved exactly (and this is just for classical mechanics and r-squared forces). Algebraic approximations or numerical simulations can do a good job, but they’re not exact.
A living object is material in an ordered state, which actively maintains its state (at least), and (better yet) is able to rearrange increasing amounts of its environment into something similar to itself, by growth (ok) or replication (best of all).
In detail, it’s an object that is…
significantly more ordered than its environment (maybe not its immediate environment, as a bacterium living inside an animal, but its broader environment, like the environment that animal lives in), and
actively maintains its ordered state (when its order is disrupted, energy is involved in (attempting to) return to its previous state), and
is able to make more of itself or something very similar to itself.
Very Universe 