A relationship between two tables in which each tuple in the first table may be related to one and only one tuple in the second table and each tuple in the second table may be related to one and only one tuple in the first table Example: DEAN has a one to one relationship to SCHOOL because each dean may be the dean of only one school, and each school may have only one dean
A mapping is one to one if no more than one element from ever maps to each element of the target set (but not all elements must be hit by the mapping) In other words, a filter is one to one if only one item can ever get mapped to a certain output Graphically, this would mean a function that is invertible