Which C.S. Lewis novel introduces the character Aslan?

The C.S. Lewis novel that introduces Aslan is The Lion, the Witch and the Wardrobe.

Published in 1950, The Lion, the Witch and the Wardrobe is the first-published book in The Chronicles of Narnia. Four English siblings—Peter, Susan, Edmund, and Lucy Pevensie—enter Narnia through a wardrobe and become involved in a conflict with the White Witch.

Aslan, a talking lion and the rightful ruler of Narnia, appears as a major character in the story. He leads the forces opposing the Witch and restores spring after her long winter. The novel's ending establishes Aslan as a continuing presence in the wider series.

The book is sometimes listed as the second volume in chronological reading orders because The Magician's Nephew describes events before it. That does not change its publication order: Lewis released The Lion, the Witch and the Wardrobe first, and it was the book that introduced readers to Aslan.