Alumnus is a Latin word meaning
male former student or
graduate student from one university or a college.
Alumni is the plural and it is used for groups of male or male and female students together. When speaking about a female student we use the feminine
Alumna and the plural
Alumnae.
These Latin words are regularly used in English maintaining the original meaning, but the pronunciation has changed.
Listen to the English pronunciation of
Alumnus,
Alumni,
Alumna,
Alumnae 
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