Real ale is the name coined by the Campaign for Real Ale (CAMRA) in 1973 for a type of beer defined as "beer brewed from traditional ingredients, matured by secondary fermentation in the container from which it is dispensed, and served without the use of extraneous carbon dioxide "This is a definition that, like Germany's Reinheitsgebot, can be seen as quite restrictive. The term" traditional ingredients" is designed, like the Reinheitsgebot, to prevent artificial preservatives or cheap adjuncts or chemicals from being used in the making or storing of the beer. The heart of the definition is the maturation requirements. If the beer is unfiltered, unpasteurised and still active on the yeast, it is a real beer; it is irrelevant whether the container is a cask or a bottle. If the yeast is still alive and still conditioning the beer, it is "real".
Cask-conditioned beers and bottle conditioned beers are often referred to as real ales, though by the terms of CAMRA'sdefinition not all cask or bottle conditioned ales are real ale; in particular, some American-style brewpubs may use collected carbon dioxide during the serving process which would disqualify them from claiming real ale status

Liverpool One Brewery only produce Real Ale unfiltered, unpasteurised and still active on the yeast

Source real ale wiki