In algebraic topology, the Bousfield class of, say, a spectrum X is the set of all (say) spectra Y whose smash product with X is zero:
The notion applies to module spectra and in that case one usually qualifies a ring spectrum over which the smash product is taken.
Iyengar, Srikanth B.; Krause, Henning (2013-04-01).
"The Bousfield lattice of a triangulated category and stratification".
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