A
winter beer is traditionally a
dark,
strong,
warming beer
available only during wintertime. Nowadays, any
seasonal beer
produced only during winter is called a winter beer, though.
Brewing winter beers is a long-standing tradition which
has its origins in the pagan celebration of winter
solstice (of course, when monasteries across Europe
started brewing their own beer the practice of brewing winter beer
instead became a celebration of the birth of Christ).