Definitions

Series - 

Sequence - 

Sign - Symbol that is agreed upon to represent, indicate, or communicate with the viewer. An exit sign, or a poison symbol. Letters in the alphabet, or shapes.

Index - An index is a feature that implies something else. An image of sunlight could be used to represent warmth. When you hear "bang" you think of what causes the noise, not the noise itself.

Symbol - Connects to large groups of words or descriptions, is interchangeable. An American flag brings to mind patriotism.