# Glossary¶

alternate
alternates
Different variants of a single cache object.
alternate selection
Selecting a specific alternate for an HTTP object.
alternate vector
A vector of alternate descriptors, instances of HTTPCacheAlt.
cache object
A resource identified by a URL. A cache object can have various forms which are called alternates. These can vary by encoding, format, or other properties. Every alternate of an object is considered semantically equivalent.
cache volume
A logical unit of cache storage.
revalidation
revalidated
Revalidation is done by sending a conditional request to an origin server to check the status of a specific alternate. Once the response is received and process the alternate has been revalidated and is presumably fresh.
slice
A version of an alternate. Slices are temporal. Different slices of an alternate are variations of the alternate through time. An alternate has more than one slice on when it is active, cache objects in storage have exactly one slice per alternate.
span
A contiguous range of bytes on a storage device.
stripe
A subrange of a span that is associated with a specific cache volume.