Q: twisted bundle definition 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? 

A: $S^2$ 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. 
