A skeletal isomer of a compound is a structural isomer that differs from it in the atoms and bonds that are considered to comprise the "skeleton" of the molecule.  For organic compounds, such as alkanes, that usually means the carbon atoms and the bonds between them.
For example, there are three skeletal isomers of pentane: n-pentane (often called simply "pentane"), isopentane (2-methylbutane) and neopentane (dimethylpropane).
If the skeleton is acyclic, as in the above example, one may use the term  chain isomerism.