Apache Lucene is an open-source search library written in Java, designed to provide powerful full-text indexing and searching capabilities. It allows developers to add search functionality to applications, enabling them to efficiently retrieve information from large volumes of text. Lucene is widely used in various applications, including web search engines and enterprise search solutions. The core of Lucene is its ability to index documents, making them searchable through a variety of query types. It supports features like relevance ranking, faceted search, and filtering, which enhance the search experience. Many popular platforms, such as Elasticsearch and Apache Solr, are built on top of Lucene, leveraging its robust search capabilities.