Leonard has used and liked Colgate toothpaste for years, so when the company introduced colgate mouthwash, he bought one immediately. This is an example of:

Th answer is: Customer fidelization or loyalty

Explanation:

This happens when a customer is willing to buy or use a product or a service from an specific brand over and over again. It usually results from a very positive customer experience and satisfaction. The customer values heavily the experience of buying and using certain brand.

This is like the Holy Grail of marketing and consumer satisfaction. Every brand's goal is to have the most loyal customers as possible.