YaCy是一个基于p2p的分布式开源免费的网页搜索引擎系统，任何人都可以用YaCy为自己创建个人的搜索门户，以实现信息的自由共享。

其核心是分布在数百台计算机上的被称为YaCy-peer的计算机进程，基于p2p网络构成了YaCy网络，整个网络是一个分散的架构，所有的peer处于对等的地位，没有统一的中心服务器，每个peer独立地进行互联网的爬行抓取，分析及创建索引库，并通过p2p网络进行共享；而且每个peer又都是一个独立的代理服务器，能够对本机使用的网页进行索引，也通过本机运行的web服务器进行查询和返回结果。

YaCy (pronounced "ya see") is a free distributed search engine, built on principles of peer-to-peer (P2P) networks. Its core is a computer program written in Java distributed on several hundred computers, as of September 2006, so-called YaCy-peers. Each YaCy-peer independently crawls through the Internet, analyzes and indexes found web pages, and stores indexing results in a common database (so called index) which is shared with other YaCy-peers using principles of P2P networks. It is a free search engine that everyone can use to build a search portal for their intranet and to help search the public internet clearly.