Support refers to the number of times the rule is satisfied relative to the total number of transactions in the dataset.

For instance:

{Milk} --> {Bread}, occurs in 1 out of 4 Transactions (in Transaction 1). Hence, the support for this rule is ¼ = 0.25 (or 25%).

{Bread} --> {Butter}, occurs in 2 out of 4 Transactions (in Transaction 1 and 4). Hence, the support for this rule is ½ = 0.50 (or 50%).