The collapse query parser and expand search component are two related features that arrived in Solr 4.7 as an alternative to Solr's Result Grouping feature (group=true). First, we'll describe these two features and then compare it to result grouping.
The collapse query parser filters search results so that only one document is returned out of all of those for a given field's value. Said differently, it collapses search results to one document per group of those with the same field value. This query parser is a special type called post-filter, which can only be used as a filter query because it needs to see the results of all other filter queries and the main query. In order to pick which document of a set is chosen to be the one returned, it by default it picks the highest scoring one, but it can be configured to choose based on the document with the highest or lowest value of a field or function query.
An excerpt of the query in action is...