Correct spelling, explanation: the problem with the spelling comes from pronunciation differences existing. While asterisk is usually pronounced as [as-tuh-risk], sometimes [s] and [k] switch places in pronunciation resulting in [as-tuh-riks]. This is why sometimes we can see a misspelling asterick – as if it was the singular form of non-existing asteriks which was created by pronunciation. However, in writing, it is always asterisk that is correct.

Definition of asterisk:
noun, a symbol (*) used in writing and printing
If you want to add extra information, write an asterisk at the end and elaborate at the bottom of the page.

Incorrect spelling, explanation: this is an incorrect form, which comes from one of two ways of pronunciation of asterisk: [as-tuh-risk] and [as-tuh-riks]. The second option may mislead to thinking, that [as-tuh-riks] is a plural form (ending with s) and therefore that the singular form is asterick. This is a mistake though. The correct spelling is asterisk, it is sometimes pronounced in two different ways.

Incorrect spelling