D&D Encounters is an official program of D&D 4e run by WotC which progresses a campaign with weekly installments. (If you're not referring to this program, use the [encounter] tag.)
D&D Encounters was the name for Wizards' entire Organized Play program during 4e. (During 5e D&D Encounters is one of the three models of play contained in D&D Adventurers League.)
This tag refers to D&D Encounters as it existed during 4e. In Encounters players walked in, sat at a table, and played through a single, thoroughly-developed, large-scale combat encounter.
From the Wizards of the Coast's (now-obsolete) D&D Encounters page:
D&D Encounters is an exciting, weekly campaign that plays out one epic encounter at a time. As you defeat enemies, solve puzzles, finish quests, and perform heroic deeds, you’ll earn Renown Points that you can use to get exclusive rewards. Each session only takes 1-2 hours to play, so it’s easy to fit your game in after school or work. And each week there’s a new and exciting challenge. Jump in anytime!
If you are asking about 5e Adventurers League's Encounters program, please use the dnd-adventurers-league tag.