The title, of course, refers to our protagonist Pi, whose full name is Piscine Molitor Patel. Pi's name has a few rich associations in the novel. For starters, there's π, the "elusive, irrational number with which scientists try to understand the universe" (1.5.41). There's also the glorious Parisian swimming pool, the Piscine Molitor, which apparently made a lasting impression on Pi's uncle, Mamaji.
So far so good: a mysterious, mathematical oddity and the favorite swimming pool of Pi's spiritual and aquatic guru. We can put check marks next to most of the major themes.
It's possible you still have a few questions. For example, why isn't the book called The Life of Pi? Did Martel forget about the article ("the")? We've got you covered. Here's an answer straight from the author:
"Like 'pi', life is not finite. And so I didn't make the title The Life of Pi: I deliberately left out the definite article. That would have denoted a single life." (Jennie Renton, "Yann Martel Interview")
Pi himself might not be immortal, but his story is. In the title, Martel reminds us both of the continuity of life and the openness of Pi's story. Meaning, the story doesn't limit itself to Pi. Ideas, people, religions – anything with the spark of life – all follow, to some degree, the pattern of messiness and depravity and hope in Martel's novel.