A compiled collection of what was every known word still in present use in the english language. It was put together in an attempt to stem the mutation of the already unstable english language and keep it in a fair condition for future generations. Though dialects have evolved since then, it has managed to maintain the grammatical and vocabulary structures of the language to a good enough point that present society can still read old language tracts without much effort. For example of success; latin, due to its spoken frequency and under developed linguistical records, after the collapse of the culture, mutated into the romantic languages of French, Spanish and Italien, while English after subsequent invasions became a hybrid of Norse (Frissian dialect) and Germanic. Only after a dictionary was written around the 17th century did the English language stabalize into its still recognizable form today.
When in doubt, look it up.