Latent heat of fusion:
            The heat energy required to change a unit mass of a substance from solid to liquid state at its melting point without a change in its temperature is called its latent heat of fusion.

It is denoted by H_f.
                                      H_f  = $\frac{( \triangle Q_{f})}{(m)}$  
   Or                   $\triangle Q_{f}$=  $m H_{f}$   …………   (i)

Ice changes at 0°C into water. Latent heat of fusion of ice is:
$3.36\times 10^{5}$  $Jkg^{-1}$.  That is, $3.36 \times 10^{5}$  joule heat is required to melt 1kg of ice into the water at 0°C.