A (indefinite article)

1. Used when referring to someone or something for the first time in a text or conversation.
2. Used to indicate membership of a class of people or things.

Origin:

Old english ān, from germanic *ainaz, from an indo-european root shared by greek oinos 'ace' and latin unus 'one'.

Examples:

1. A dog barked outside.
2. She is a teacher at the local school.
3. A laptop is on the desk.
4. He wants to be a doctor when he grows up.
5. I found a penny on the ground.