1.  timer(n = noun.artifact) - a timepiece that measures a time interval and signals its end;
is a kind of horologe, timekeeper, timepiece
has particulars: chronograph, parking meter, stop watch, stopwatch
Derived forms verb time₁, verb time₃

2.  timer(n = noun.person) timekeeper - (sports) an official who keeps track of the time elapsed;
is a kind of official
Derived form verb time₁

3.  timer(n = noun.artifact) - a regulator that activates or deactivates a mechanism at set times;
is a kind of governor, regulator
Derived form verb time₄

1.  A timekeeper; especially, a watch by which small intervals of time can be measured; a kind of stop watch. It is used for timing the speed of horses, machinery, etc. [1913 Webster]

2.  In electric ignition, an adjustable device for automatically timing the spark. [Webster 1913 Suppl.]

timer, n.
1 a person or device that measures or records time taken.
2 an automatic mechanism for activating a device etc. at a pre-set time.

egg timer, old timer, two timer