(covers information from several alternate timelines)
Horatio Nelson was a vice admiral of the Royal Navy in the early 19th century on Earth. He led his fleet to victory against a larger combined French and Spanish force at the Battle of Trafalgar, the decisive naval engagement of the Napoléonic Wars. Nelson was wounded in the battle, dying below decks on his flagship HMS Victory.
A 22nd century Earth starship, the HMS Lord Nelson, was named after him. (TNG: "Up The Long Ladder")
A monument, called Nelson's Column, was erected in London to honor Nelson and still stood in 2259 of the alternate reality. (Star Trek Into Darkness)
While deciding whether or not to retreat from a Borg cube laying in wait for the USS Enterprise-D in 2366, Captain Picard thought of Nelson. Talking with Guinan, he recalled the Admiral's tour of the Victory preceding his final battle. (TNG: "The Best of Both Worlds")