Original Dungeons & Dragons refers specifically to the 1974 game co-created by E. Gary Gygax and Dave Arneson, published by Gygax's Tactical Studies Rules. It was sold as a collection of three pamphlets in a woodgrain box. Later would come more expansion pamphlets (including those for Gygax's "Greyhawk" campaign and Arneson's "Blackmoor") as well as articles in the fledgling Dragon magazine.
The game originally used the man-to-man combat rules from the TSR wargame Chainmail 3rd Edition, and the board from the Outdoor Survival board game, though neither are required to play. In fact, many of the alternate systems proposed in the D&D pamphlets themselves would become standard, though the Chainmail rules were still imported for handling distance measurement and mass combat, and the like.
In 1978 a "collector's edition" was printed and sold in a white cardboard box and given the nickname "whitebox D&D". Since it is fundamentally similar to the original, it is often referred to as OD&D as well.
This game has also been called "0th Edition", "First Edition" or "1e". The latter terms can be confusing, as they are frequently used to refer to the first edition of Advanced Dungeons and Dragons as well, and occasionally the Basic D&D revisions from the 1980s.