Correct spelling, explanation: this word came into English in late Middle English, but it originates in Latin. Nowadays, it has quite a wide range of meanings and is used in different contexts. The only correct spelling of the word is with a single -d letter and a double -s letter.

Definition of admission:
1. noun – which refers to an unwilling act of acceptance, to money you need to pay for entering a place, or permission to get into a place:
Leaving the pitch just like that in the middle of the game was his admission of defeat.
The admission charge to enter the exhibition is $50.
Admission to the gathering is only for the official members.

Expressions with admission:
1. admission of liability – when a person or an organization accepts legally that they are responsible for somebody’s harm,
The admission of liability cost the company a lot of money and losing its good reputation.
2. to deny admission to – to refuse admission to,
Despite the fact that I had a valid visa they denied my admission to the country.
3. to gain admission – to get permission to enter some place,
I had to go through a long formal process to gain admission to the organization.

Incorrect spelling, explanation: both learners of English and its native speakers often believe that the word admission combines two different words, namely, add and mission. As a result, a common mistake in the spelling is to double a -d letter, which gives the form addmission. It is wrong, though, so do not use it.