“Contraction in supply” means fall in quantity supplied due to fall in price of good only, other factors remaining unchanged. For example, fall in supply from OQ1 to OQ2 is on account of fall in price from OP1 to OP2.
“Decrease in supply” means fall in supply due to factor other the price of the good. For example, fall in supply from OQ1 to OQ2 is at the same price. It means that some factor has caused this change.
