Incorrect spelling, explanation: this form is wrong because there should be one more h in the spelling, as in eight, which is the core word here. Eight + -th (suffix for ordinal numbers) is eighth (suffix merge with the word and we don’t double the consonant t). This is why the correct form, in this case, is eighth, not eigth.

Correct spelling, explanation: eighth is an ordinal number formed from the cardinal number eight. To create an ordinal number form we add the suffix -th to the cardinal form. In the case of eight, the word already ends with consonant t so we just need to add h to receive an ordinal number. Therefore eighth is the correct form and eigth is incorrect as there is a lack of one consonant h.

Definition of eighth:
numeral, the ordinal number between seventh and ninth
Have you heard that Judy is pregnant with her eighth child?!
– How did he score in the contest?
– Not bad, he was seventh or eighth, I think.