Double threat
A double threat means that a player has two different options for achieving a specific goal, usually connecting two groups of pieces. A double threat is a special case (and the most common example) of a multiple threat.
A double threat is also sometimes called miai, a Japanese term taken from Go.
Examples
The most basic example is the bridge as seen below. The starred cells form a double threat: if Blue plays in one of them, Red secures the connection by playing in the other.
The following is another example. Red's two groups are connected by double threat at the two cells marked "*".
If Blue moves at one of the cells marked "*" or its neighbors, Red can respond at the other one, thus guaranteeing a connection between the two groups.
A ladder escape fork can also be regarded as a kind of double threat.
