May is Mild Month, as declared by the UK's Campaign for Real Ale, or CAMRA.
Milds are black to dark brown to pale amber in colour and come in a variety of styles from warming roasty ales to light refreshing lunchtime thirst quenchers. Malty and possibly sweet tones dominate the flavour profile but there may be a light hop flavour or aroma. Slight diacetyl (toffee/butterscotch) flavours are not inappropriate. Alcohol levels are typically low.
Pale milds tend to have a lighter, more fruity aroma with gentle hoppiness.
Dark milds may have a light roast malt or caramel character in aroma and taste.
Scottish cask beers may have mild characteristics with a dominance of sweetness, smooth body and light bitterness.
Original gravity: less than 1043
Typical alcohol by volume: less than 4.3%
Final gravity 1004 - 1010
Bitterness 14 - 28 EBU
Chameleon-like (or based upon what your definition of is is), descriptions of what Mild Ale actually is can differ —sometimes diametrically— based upon informed opinion, historical research, and commercial practice.
Ron Pattinson's book Mild! would be a good place to start.