The rule for "a" or "an" is a phonetic rule, not an orthographic rule. The rule is that the indefinite article "a" is used before a singular countable noun and becomes "an" when the following word begins with a vowel sound. an apple a red apple a banana an old banana "a" become "an" when the following word begins with a silent "h." an hour an hourly wage a hat [not pronounced with a silent "h"] a hotel / an 'otel [depends on the age and region of the speaker] "a" is used before words that start with "u" and "eu" when they are pronounced with an initial "y." a university [pronounced "yuniversity"] a universal truth [pronounced "yuniversal"] an ugly dog [not pronounced "yugly dog"] a European language [pronounced yuropean] More information: https://grammar.collinsdictionary.com/easy-learning/the-indefinite-article [excerpt] They joined a historical society. They joined an historical society. (old-fashioned English) They were staying at a hotel. They were staying at an hotel. (old-fashioned English)

I was taught it is called a semi-consonant (or semi-vowel). Same goes for "university", "youth", "yell", etc.

I have got a uniform.

A uniform. Even though it starts with a vowel, the sound is a consonant ("y").