Direction Cosines:

Let α, β, γ are the angles which a vector OP makes with the positive directions of the coordinates axes ox, oy and oz respectively.

Then cosα, cosβ and cosγ are known as the direction cosines of OP and are generally denoted by the latters l, m, n

i.e. l = cosα, m = cosβ, n = cosγ

The angles α, β, γ are known as the direction angles where 0 <= α, β, γ <=0 

Direction Ratios:

Let l, m, n are the direction cosines of a vector r and a, b, c are the three numbers such that

l/a = m/b = n/c

Then a, b, c are known as direction ratios or numbers of vector r.