What is the difference between fertilization and fertilisation?

Dear there is no difference. Infact both fertilization or fertilisation means to the fusion of gametes.

Now if you are still feeling unsatisfied, i would like to tell you that some words are spelt differently according to british and american dictionaries. So thats why you may find it different at different references.

Thanks

The alternative form of fertilisation is fertilization. It is only a difference of spellings.

Fertilization is American English whereas Fertilisation is British English.