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Alarm -  a device that sounds a signal at a pre-set time. A subdial is used to set the alarm.
               a hammer is used to produce a sound at a pre-set time.

Altimeter - A device that determines altitude by responding to changes in barometric pressure.

a.m. - ante meridian (before noon). From the latin ante - before, medius - middle, 
diem - day.

Analog Display – 

    The time display is shown by hands and a dial. 
    The opposite to digital display.

Anchor - moving side to side the anchor allows the final wheel (escape wheel) 
                 to rotate one cog at a time. 
                This process produces the ticking sound of a mechanical watch.

Anti-magnetic watches - watches that are not affected by magnetic fields.

Anti-reflection - film on the crystal to eliminate light reflection.  Improves readability.

Aperture – A small opening in the dial that displays certain information such as date, day, month or moonphase.

Applique - numerals or symbols cut out and stuck to a dial.

Arabic numerals - 1, 2, 3, 4, 5, 6, 7, 8, 9, 0

Arctic Circle - This line is located 23.5° south of the North Pole.

Armored crystal - an armored crystal is not some kind of specially treated plastic. It is an otherwise plain, plastic crystal, meant to be installed by compressing it to fit into a groove in the face of the watch case, as opposed to cementing it in place. They are installed by squeezing their outer diameter enough to fit inside the groove. Tools that do this apply pressure from all around the edge at one time, raising the center of the crystal while they reduce the outer diameter of the crystal. What distinguishes an armored crystal from common snap crystals is a ring of metal at the base, meant to hold the crystal even more tenaciously in place.

Assembling - The process of fitting together the components of a movement.  Formerly done entirely by hand but now mainly automated.

Asthomometer -  dial on a chronograph for measuring the rate of respiration.
                                

ATM - the water pressure rating of a watch. ATM means atmosphere and is equal to 10 meters of depth. (1 meter = 3 feet)
            1 ATM = 10 meters
            5 ATM = 50 meters

Automatic watch – A watch that has the mainspring wound by the movement of the wearer's wrist rather than winding a stem. Also referred to as a "self-winding" watch. A weight (the rotor) is turned by the motion of the wrist thus winding the mainspring. The energy generated by the movement of the rotor is transferred into the mechanical energy of a spring and then to the watch movement. If an automatic watch winds down most of them can by wound by hand or shaken to get it started again. The system was invented for pocket watches in Switzerland in the 18^th Century by Abraham-Louis Perrelet. This system was successfully adapted to the wristwatch in 1923 by John Harwood, an English watchmaker.

Auto Repeat Countdown Timer - A countdown timer that resets itself as soon as a preset time has elapsed.  It repeats the countdown continuously until a button is pressed to stop the function.