An operation in a version control system or a special way of creating a shared library. (In order of the question counts in stackru)

Version Control Systems

In some source control system (also known as Version Control Systems o vcs) that support branches a rebase is one of the ways of integrating the changes of one branch into another by changing the "father" or starting point of a branch.

Git

See git-rebase.

Darcs

Support it's still in an early status (March 2013)

Mercurial

There is a Rebase Extension for mercurial as noted in " How to do a rebase like git's rebase"

Subversion

There is no rebase support but using merge --reintegrate might be considered as a similar operation as mentioned in " Subversion rebase?" and " Subversion Branch Reintegration v1.5"

Shared Libraries

Creating a shared library to avoid conflicts with other libraries in the usage of virtual memory. ( Wikipedia)