A geocache series is a group of caches with a related theme. As opposed to a multi-cache, these caches tend to stand alone individually.
Geocaches with a given series will tend to have some element in common. Usually this is seen in the category of location involved, such as in cemeteries or urban parks. Sometimes the similarity may lie in the type of container used. Still another category may simply be in the cache information, which is often the case in a series of puzzle caches.
Usually each cache of a given series will be identified with a similarity in title, with each individual cache having an indexed number and/or unique sub-title. Often each cache will stand on its own, independent of other caches in the series; however, a cache series may be interrelated in some fashion. One example is a series where some caches provide information for another cache in the series, such as finding a given number of caches with the same amount of clues to locate one or more other caches.
Geocache series often have one geocaching team responsible for all of the caches in the series. However, there are series that incorporate multiple hiders. Series hidden by a single team tend to be located within a localised geographic location. Series with multiple hiders may extend over a large geographic region, emcompassing caches in different states/provinces or even different countries. The Project APE set of caches listed on Geocaching.com is among the most well-known geocache series.