Dimension: What is a Dimension in Web Analytics?
In web analytics (including Google Analytics), a Dimension is an attribute or characteristic such as Browser Client Type or a URL such as Exit Page. In most analytics programs (including standard Google Analytics reports) Dimensions will typically be represented in columns. Although Metrics and Dimensions have value as stand-alone pieces of data when we combine them into data segments we typically get exponential value (and insight!) from the data.
A full reference including compatibility is available from the Google Developer Forum - "Dimensions & Metrics Reference"