Correct spelling, explanation: an English noun – signature – has its origin in the Latin word signare, which had the meaning of sign or mark. In the times of Medieval Latin signature got the form of signatura and meant sign manual. Signature entered English in the 16th century and in modern English, it is a noun.

Definition of signature:
1. noun – the way you write your name on official documents, etc.,
I have never seen this document and it’s not my signature down here.

Collocations with signature:
Some common collocations include: collect signatures; bear a signature, and witness a signature.
Your petition won’t be considered until you collect at least one thousand signatures.
The document bears her signature so she is the one who will take the responsibility for that.
To confirm the validity of your last will, your signature needs to be witnessed by two people.

Incorrect spelling, explanation: the word signature is quite troublesome when it comes to its spelling. Many English users use the -i letter instead of the -a letter, which results in an incorrect form signiture. The only right form is spelled with -a, which gives signature.