What is "blackboard bold"

n. A typeface used for representing bold characters, mainly used in mathematical texts, that allows these characters to be easily distinguished from non-bold characters when written.

Blackboard bold is a typeface style that is often used for certain symbols in mathematical texts, in which certain lines of the symbol (usually vertical or near-vertical lines) are doubled. The symbols usually denote number sets. One way of producing blackboard bold is to double-strike a character with a small offset on a typewriter. Thus they are also referred to as double struck.