SAN FRANCISCO (Reuters) - Ask.com, the Web search unit of IAC/InterActiveCorp, has closed an all-cash deal to acquire Lexico Publishing Group LLC, the owner of popular reference sites Dictionary.com and Thesaurus.com.

Lexico of Long Beach, California, was founded in 1995, starting with Dictionary.com. A year later Lexico introduced Thesaurus.com and in 2002 began Reference.com. For users who know what they mean but can't recall the term, it also offers a Reverse Dictionary (http://dictionary.reference.com/reverse/).

Ask.com said the deal expands its monthly audience by around 15 million users to 145 million, an increase of 11 percent. Based on March traffic data from comScore Inc, that would make Ask.com the ninth-largest Web property globally.

Dictionary.com is the fifth most-visited reference site among U.S. Web users, according to data from online audience measurement site Hitwise Inc. Reference.com is 39th most visited and Thesaurus.com ranks 47th.

The influence of Ask.com within e-commerce conglomerate IAC/InterActiveCorp is growing as New York-based IAC plans to spin off four of its largest businesses and focus on Web media and advertising in coming months.

Terms of the merger were not disclosed. The transaction was originally announced on May 15.

(Reporting by Eric Auchard; Editing by Steve Orlofsky)