1,4-Addition Definition:

A reaction involving the addition of two groups to a conjugated  π system in which one group is installed at the C1 position and the other group is installed at the C4 position.

1,4-Addition Explained:

In a 1,4-addition reaction, two atoms are added to the first carbon position (C1) and the fourth carbon position (C4) in a conjugated π.

The resulting product is called a “1,4-adduct”