Let $H\subset G$ be a subgroup and $\pi:P\to B$ be a principal $H$-bundle.
$G$ has a left $H$ action and one can define a principal $G$-bundle $\pi':P\times_H G\to B$ where $P\times_H G$ is quotiening out the diagonal $H$-action of $P\times G$
This latter bundle $\pi'$ is called the extension of the structure group from $H$ to $G$ of the bundle $\pi$, I guess.
There also exists another term reduction of the structure group. Is this just the ''dual''? i.e. is $\pi$ the reduction of the structure group from $G$ to $H$ of the bundle $\pi'$?