Correct spelling, explanation: the word admission has its origin in the Latin verb admittere. The word entered the English language in the times of late Middle English. Grammatically admission is classified as a noun and is used in both formal and informal contexts.

Definition of admission:
1. noun – agreeing, often unwillingly, that something is true;
He didn’t answer my question, which I understand as an admission of guilt.
2. noun – the money that has to be paid in order to enter someplace;
The admission charge to this event is 10 pounds.
3. noun – permission to get into someplace;
Admission to the building is with an ID only.

Collocations with admission:
Some common collocations with admission are: free admission, apply for admission, refuse somebody admission;
There’s going to be free admission to the concert so for sure there will be a lot of people.
In order to apply for admission to the university, you need to hand in all the documents by the end of August.
You are under 18 so we have to refuse your admission to the club.

Incorrect spelling, explanation: many users of English mistakenly think that the word admission is a combination of two words – add and mission – as a result, a common mistake is to spell the word as addmission, which is wrong. The only correct spelling of the word is admission.