Given two sets of attributes X and Y, a functional dependency between them is denoted by X -> Y. The constraint says that, for any two tuples u1 and u2, if u1[X] = u2[X], then u1[Y] = u2[Y].

Let w be a tuple distance function on R(z1:Z1,.., zn:Zn), t a maximum distance threshold, x=(x1,.., xn) and y=(y1,.., yn) two tuples in R, we say that x is similar within t to y with respect to w, denoted by x =_w(t) y, iff w(x, y)=< t.