 For the 2012 revision, we are grateful to Kurt Sylvan for extremely detailed and constructive comments on multiple drafts of this entry. Thanks also to an anonymous referee for additional helpful suggestions. For the 2017 revision, thanks to Clayton Littlejohn, Jennifer Nagel, and Scott Sturgeon for helpful and constructive feedback and suggestions. Thanks to Ben Bayer, Kenneth Ehrenberg, and Mark Young for drawing our attention to errors in the previous version.