Two goods are substitutes if a rise in the price of one causes an increase in demand for the other. This substitute relationship arises because the goods perform a similar function or serve a similar taste. However, we can generally think of a whole spectrum of substitution possibilities for a particular good, and so we aften refer to ‘dose’ substitutes or ‘weak’ substitutes. For example, a Pepsi would be a dose substitute for a Coke, but a Martini would not. The doser are the available substitutes for a product, the greater, other things being equal, will be its price elasticity of demand.
Reference: The Penguin Dictionary of Economics, 3rd edt.