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 users to find information quickly and efficiently. Lucene is widely used in various applications, from web search engines to enterprise search solutions. The library supports various features, including text analysis, scoring, and ranking of search results. It can handle large volumes of data and is highly customizable, making it suitable for different use cases. Many popular projects, such as Apache Solr and Elasticsearch, are built on top of Lucene, leveraging its robust search capabilities.