What it is: The vehicle used to deliver a campaign to your audience. Common media includes email, pay-per-click (PPC), tweets and organic search. 

Keep in mind: Other analytics tools refer to the "medium" differently. For example, SiteCatalyst requires you to use SAINT to group certain campaigns into buckets, which essentially serves the same purpose as a campaign medium in Google Analytics.

Why it's important: Understanding and defining your campaign's medium can help you gauge success between your different conversion efforts. Imagine you have two running: one effort through email, and one through PPC. Now you can compare metrics (such as visitors' length of stay) across media.

The bottom line: Comparing the success of your campaign between media will help you understand each one's contribution to your KPIs—in turn, you can make more informed marketing decisions that will help you increase both revenue and conversion rates.

This post is part of an ongoing Clickipedia series, defining all the terms you need to know for conversion rate optimization. Keep checking back each week to learn more, and don’t forget to check out the whole series.