Question

I wanted to know what is the exact definition of a seed crystal

Answer 1

Seed crystals are tiny single crystals made to dissolve in a solution (or melt) from which larger crystals are drawn either by precipitation, centrifuging or drawing up from a melt. Seeds have the same chemical composition and crystalline structure as that of the larger crystal.
Large crystals are grown on a preferred geometer (normally the most densely packed) plane of the seed.
Such seeding is a common practice in semi-conductor industries and making artificial single crystal gemstones like silicon, germanium, sapphire, diamond, zirconia, etc.