A descriptive attribute or characteristic of data. Browser, Landing Page and Campaign are all examples of default dimensions in Analytics.
A dimension is a descriptive attribute or characteristic of an object that can be given different values. For example, a geographic location could have dimensions called Latitude, Longitude, or City Name. Values for the City Name dimension could be San Francisco, Berlin, or Singapore.
Browser, Exit Page, Screens, and Session Duration are all examples of dimensions that appear by default in Analytics. Dimensions appear in all of your reports, though you might see different ones depending on the specific report. Use them to help organize, segment, and analyze your data.
Analytics also lets you create custom dimensions to hold additional types of data you send via the tracking code, or by using Data Import, or by using the Analytics API.