Local Routing Number, a ten digit phone number used to route phone calls between two telephone exchanges
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Luminous red nova, a stellar explosion thought to be caused by the merger of two stars
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Lateral reticular nucleus, a nucleus of the medulla oblongata involved with co-ordinating baroreceptor signals to control arterial blood pressure
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LORAN, a terrestrial radio navigation system originally known as LRN for Loomis Radio Navigation