Correct spelling, explanation: spelling of the word asterisk is commonly mistaken since this word is sometimes pronounced [as-tuh-riks], which may be understood as a plural form. Therefore one may assume there is also a word asterick (a singular one). However, asterisk is not plural and this spelling is correct.

Definition of asterisk:
noun, a symbol (*)
We usually add an asterisk after a word if we want to add some extra notes, not directly related to the subject.

Incorrect spelling, explanation: there is no such word as asterick. The mistake probably comes from two ways of spelling asterisk, which are [as-tuh-risk] and [as-tuh-riks]. The second one, mistaken as a plural form, may point to the existence of a singular form, however, this is not the case. Asterisk is the only correct spelling.

Incorrect spelling