A chronograph is a watch that tells the time and records elapsed time on demand. Basically, it is a watch with a stopwatch integrated into the movement.
It is not necessarily a chronometer and the two terms should NEVER be confused. For a watch to be a chronometer it must meet a certain standard in terms of accuracy.
A chronometer is a watch of exceptional isochronism, measured by a series of positional and situational tests and overseen by an independent body, such as COSC (Control Officiale Suisse de Chronometers).
The COSC test lasts for 15 days and tests the watch’s timekeeping abilities in a variety of positions common during wear, and in varying temperatures to ensure the watch will be a consistent timekeeper in all states.
The standard set by COSC requires the mean rate of a watch to be within +6 seconds and -4 seconds per day.
REMEMBER: Chronometer and Chronograph are separate terms, but a chronograph can be a chronometer if it passes the requisite tests.