I have often heard people talk about, say, "the" twisted $S^2$-bundle over $S^2$.
My question is, what do they mean by a twisted bundle? I know that in the above example any $S^2$-bundle over $S^2$ is either $S^2 \times S^2$ or a unique non-trivial $S^2$-bundle over $S^2$. But is that how it is defined? Is a *twisted bundle* just a non-trivial one, or is it a specific bundle among the non-trivial ones? 



twisted bundle definition
$S^2$ bundles over $S^2$ (smooth, PL, topological) are in bijective correspondence with homotopy-classes of maps:


$$ S^2 \to BSO\_3 $$


which up to homotopy is


$$\pi\_2 BSO\_3 \simeq \pi\_1 SO\_3 \simeq \mathbb Z\_2 $$


So there's precisely two non-isomorphic $S^2$-bundles over $S^2$. You can view the non-trivial $S^2$-bundle as the fibrewise one-point compactification of the vector bundle over $S^2$ whose Euler class is $1$. That vector bundle has fairly standard constructions. 


