Incorrect spelling, explanation: some users of English think that the word admission is a combination of add and mission. As a result, they make the mistake of spelling the word as addmission. This form, however, is wrong so next time you write the word, make sure you do it right.

Correct spelling, explanation: this Middle English noun came from the Latin verb admittere. In contemporary English admission is a noun widely used in both formal and informal contexts. The spelling of the word is confusing for many users of English as they believe that there is a double -d in the word while there is only a single one.

Definition of admission:
noun – the act of agreeing with something, but often not willingly; the money you pay to enter a place; when you’re allowed to enter a place.
Paying the money without any objection was his admission of guilt.
The admission to enter the museum is only $5.
Admission to the office is only with a valid company card.

Expressions with admission:
Admission often collocates with the verb to make, the preposition to, and adjectives emergency or free:
You have to make a clear admission that it was your fault.
Admission to the exhibition is limited to 100 people a day.
Emergency admission to the hospital saved her life.
It was free admission so the place was full.