Bousfield class
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.
Sources and credits
This article is adapted from the Wikipedia article “Bousfield class”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.
Fathomly is not affiliated with or endorsed by the Wikimedia Foundation. Spotted a problem? Tell us.