You'll hear this in the context of radio transmission, for example when you need to network two buildings by a radio link. The farther apart the antennas, the bigger the Fresnel zone gets - it's the area clear of obstructions that you need for a clean signal. See Fresnel Zone Clearance. A calculator: https://www.radiolan.com/fresnel.html
Thanks to Jeff Liebermann for putting these links in a newsgroup post.
The term originated from optics, specifically the Fresnel lense used in lighthouses.
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