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Explanation: One sigla is only one letter. An anacronym is a group of SIGLAS standing for individual words & the acronym is usually pronounced as a word (radar, for example). Otherwise, a groups of siglas that is not usually pronounced as a word is an initialism (FBI, CIA,for instance)
So your translation of sigla should be "capital letter".
This is from the URL below:
Según el Diccionario de la Lengua Epañola de la Real Academia:
Sigla es la letra inicial que se emplea como abreviatura de una palabra. S. D. M. son las siglas de Su Divina