Bard is a medieval Gaelic word for a specific class of poet that evolved to mean any epic storyteller. Shakespeare was originally known as the Bard of Avon, for his birthplace. Today he is known as the Bard because he is considered the greatest storyteller of all time.
Little is known about William Shakespeare's early life, but it is likely that he did not earn his nickname until after death. In his time, Shakespeare was considered a less popular playwright than his peers Christopher Marlowe and Ben Jonson. But seven years after his death, two of his friends published the First Folio, which was the original anthology of his plays. This began to make Shakespeare's work the most well-known and easily reproduced. He eventually came to be known as the Bard of Avon because he was born in the town of Stratford-upon-Avon.
Shakespeare's popularity increased with the expansion of the British Empire. His plays became synonymous with the English language, and his importance to British culture grew exponentially. David Garrick, a highly popular actor in the eighteenth century, was a devoted fan of Shakespeare. He is credited with inspiring the first major wave of Shakespeare mania. By the time the Royal Shakespeare Company was founded in 1875, Shakespeare was officially known as the Bard.