A dimension is hit-based if it changes from hit to hit within a single visit. For example, content groups, products, and ads are all hit-based dimensions because it is possible for each a visitor to click in a different one for each hit. Hits, page views, and click throughs are all hit-based measures.
A dimension is visit-based if it does not change or occur multiple times within a single visit. For example, each visit typically has only one referring site and one geographical location such as a country. Visits, visit duration, visit depth are all visit-based measures.
A dimension is visitor-based if it does not change for an individual visitor, even across visits. For example, each visitor has only one initial campaign or initial search engine. Visitor-based measures include unique visitors and unique buyers.
If you select a visitor-based dimension, you can safely use visitor-based, visit-based and hit-based measures. If you select a visit-based dimension, you can use visit-based and hit-based measures. However, you can only use hit-based measures with a hit-based dimension. Measuring a visit-based or hit-based dimension using a visitor-level measure such as unique visitors produces inaccuracy in proportion to the percentage of repeat visits within the chosen time frame. For example, if you create a report using content groups (hit-based) as the dimension and unique visitors (visitor-based) as the measure, your data becomes increasingly inaccurate over time as the same visitor returns to the site and views different content groups.