Capitalization (or capitalisation — see spelling differences) is writing a word with its first letter as a capital letter (upper-case letter) and the remaining letters in lower case. This of course only applies to those writing systems which have a case distinction. The term is also used for the choice of case in text.
Conventional writing systems (orthographies) for different languages have different conventions for capitalization.
The systematic use of capitalized and uncapitalized words in running text is called "mixed case". Conventions for the capitalization of titles and other classes of words vary between languages, and to a lesser extent between different style guides.
In some written languages, it is not obvious what is meant by the "first letter": for example, the South-Slavic digraph 'lj' is considered as a single character for collation purposes (a situation which occurs in many other languages) and can be represented by a single Unicode character, but at the start of a word it is written 'Lj': only the L is capitalised. In contrast, in Dutch, the digraph 'ij' (known as the Dutch 'Y') is fully capitalized in initial position, for instance in the name of the city of IJmuiden.