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In the first six books, it was just widely feared. During the seventh book, however, it was made a taboo. If someone said "Voldemort", all protective spells and enchantments were broken and the Death Eaters or Snatchers would be able to apparate to your current position.

In the years of 1997-1998, after the Death Eaters infiltrated the Ministry of Magic and placed their puppet Pius Thicknesse in charge as minister. Voldemort placed a taboo curse on his name. that way, if any witch or wizard said the name "Voldemort" all enchantments around them would break, letting Death Eaters and Snatchers apparate to that location.

Before, not saying "Voldemort" and calling him "you-know-who" or "he-who-must-not-be-named" would be a sign of fear because he is the darkest wizard since Gellert Grindewald.