The
interaction between the top of a
piston's
travel in a
cylinder (
aka top dead center) and when the
spark plug fires. It also determines where the
piston is located in the
cylinder when the
valves open and close. Poorly
adjusted
timing will cause
increased fuel consumption,
crappy
throttle response,
much less
power and
worse emissions to boot. Really bad
timing can
destroy your
engine, like if a
valve and
piston both try to
occupy the same
space.
Bad.
The crankshaft (which controls the pistons) and camshaft(s) (which control(s) the valves) are linked together through a timing belt or timing chain around the crankshaft and camshaft pulleys. There are often markings on a housing and on the crankshaft pulley for a good reference as to when one is near TDC on a particular cylinder. The timing is expressed as degrees before or past TDC.
A common
trick to get a
vehicle to pass
emissions inspection is to bump the
timing back (
aka retard the
timing). With now
retarded
timing, the
engine makes less
power and usually much better
emissions.
Advancing the
timing usually results in an increase of power and much worse
emissions. This only works for small amount of change - too much and you'll have a
piston/
valve crash (the engine wouldn't even
run at this point, but the
possibility of
catastrophic damage is very
real).