The Sicilian Defence is a chess opening that begins with the moves: 1. e4 c5
The Sicilian is the most popular and best-scoring response to White's first move 1.e4. "Indeed, most statistical surveys suggest that 1.d4 is the most successful first move for White, but only because 1...c5 scores so highly against 1.e4."  New in Chess stated in its 2000 Yearbook that of the games in its database, White scored 56.1% in 296,200 games beginning 1.d4, but a full two percentage points lower (54.1%) in 349,855 games beginning 1.e4. "The main culprit responsible for this state of affairs" was the Sicilian, which held White to a 52.3% score in 145,996 games. At the elite level (> 2500 FIDE rating), 1...e5 scores slightly better, mainly because of the difficulty in breaking down defences such as the  Berlin Wall.
One sixth (17%) of all games between grandmasters, and one quarter (25%) of the games in the Chess Informant database, begin with the Sicilian. Almost one quarter of all games use the Sicilian Defence.  
Grandmaster John Nunn attributes the Sicilian Defence's popularity to "its combative nature; in many lines Black is playing not just for equality, but for the advantage. The drawback is that White often obtains an early initiative, so Black has to take care not to fall victim to a quick attack." Grandmaster  Jonathan Rowson considered why the Sicilian is the most successful response to 1.e4, even though 1...c5 develops no pieces, and the pawn on c5 controls only d4 and b4. Rowson writes:
To my mind there is quite a straightforward explanation. In order to profit from the initiative granted by the first move, White has to make use of his opportunity to do something before Black has an equal number of opportunities of his own. However, to do this, he has to make 'contact' with the black position. The first point of contact usually comes in the form of a pawn exchange, which leads to the opening of the position. ... So the thought behind 1...c5 is this: "OK, I'll let you open the position, and develop your pieces aggressively, but at a price – you have to give me one of your center pawns." 
The earliest recorded notes on the Sicilian Defence date back to the late 16th century by the Italian chess players Giulio Polerio and Gioachino Greco.