Fertilised egg= When the female reproductive organ that is ovum / egg fuses with male reproductive organ that is sperms the ovum/egg get fertilised . For fertilisation of egg sperms are needed.

Unfertilised egg= When egg is not fertilised because it does not fuses with sperms is a unfertilised egg. 

We should eat fertilised egg  [egg of a hen] 
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why we should eat the fertilized egg it may cause the death of a upcoming embryo
and doing it more may result in extinction of the species