Correct spelling, explanation: this word of Latin origin appeared for the first time in late Middle English. Its meaning has evolved since then and today it is both a countable and uncountable noun used in a variety of formal and informal contexts. When it comes to the correct spelling there is a single -d letter, but a double -s letter.

Definition of admission:
1. noun – when somebody agrees or accepts that some fact is true, usually unwillingly,
Not saying a single word by him was an admission of guilt.
2. noun – a charge you pay to enter some place,
The admission charge to enter this club is 50 pounds.
3. noun – when you are permitted to enter a place,
Sorry, but admission to this party is only by a formal invitation.

Collocations with admission:
The word admission is often followed by the prepositions of/to:
The admission of new students will be the first part of the celebration.
Admission to this country is only by a valid visa.
A few other common collocations include: to apply for admission, to refuse admission, admission price, admission charge, admission process.

Incorrect spelling, explanation: it is quite a common mistake to double a -d letter in the word admission, which gives the incorrect form addmission. The mistake probably comes from the fact that some users of English believe that the word admission is a combination of two English words – add and mission. However, it is not and the only correct spelling is admission.