I

Ides of March - From the ancient Roman calendar. It is the 15th day in March, May, July and October.  It is the 13th day in the other months. It was on March 15, 44 B.C. that the Roman dictator Julius Caesar was assassinated, hence, "Beware the Ides of March".

Incabloc - A shock-proofing system for mechanical watches in use since 1933.  It is the best known shock absorber for watches.

Integrated Bracelet - A watch bracelet that is incorporated into the watch case.

International date line -  whenever we cross it the date advances one day (going west) or goes back one day (going east).

Isochronism - equal periods of time.  A watch's ability to maintain it's rate as the mainspring unwinds.