The word is used in language study to mean 'any mark used in writing or printing to distinguish one letter, or the pronunciation of a letter, from another'. For most people, there is little if any difference between a diacritic and an accent, in the same sense - i.e. something written down. Pedants prefer to call these marks of pronunciation of vowels or consonants 'diacritical accents', to distinguish them from other meanings of the word accent.
Common examples of diacritical marks in European languages include the umlaut (¨ as in ä or ü) from German, and the tilde (~ as in ñ) from Spanish. In many Slavonic languages there are also markers to show particular consonants: , č and ň for example.