Define unit vector, null vector and position vector?

Unit Vector : A vector which has unit magitude is called a unit vector.
$\widehat{\mathrm{A}}=\frac{\mathrm{A}}{\left|\overrightarrow{\mathrm{A}}\right|}$
Null vector : A vector Whose Magnitude is Zero. Then the vector is Known as Null vector.
Position vector : The position of the particle in the space explained by vector is called position vector.
If x, y, z are co-ordinates of a point ‘p’ and ‘o’ is the origin, then
$\overrightarrow{\mathrm{OP}}=\overrightarrow{\mathrm{r}}=\mathrm{x}\hat{\mathrm{i}}+\mathrm{y}\hat{\mathrm{j}}+\mathrm{z}\hat{\mathrm{k}};\left|\overrightarrow{\mathrm{r}}\right|=\sqrt{{\mathrm{x}}^2+{\mathrm{y}}^2+{\mathrm{z}}^2}$