The definition of “saint” is a bit of a moving target. There are a few different meanings depending on the context.
Generally speaking, Catholics use the word “saint” to refer to any person in heaven. The Church has a process (called canonization) through which She discerns whether someone can truly be considered to be in heaven and thus publicly given the title of Saint.
Another use of the word among Catholics would be the concept of the “Communion of Saints” which refers to all the souls in heaven, purgatory, and everyone on earth united to them through Jesus Christ. Thus while a very holy person on earth does not have an official title of saint they can certainly be considered to be a part of the Communion of Saints.