Including: Favourable Site
An Alloy will start to solidify at any place in the liquid which is energetically "favourable" - i.e. makes it easier for the solidification to begin.
These sites are generally either impurities in the mixture (especially oxides which have very high melting temperatures) or the surface of the mould or container itself. The sites are "heterogeneities" - that is, not consistent with the rest of the liquid or, to use another big word, not homogeneous.
In solid state reactions grain boundaries are often favourable nucleation sites.
Associated terms: Alloy, Homogeneous, Heterogeneous