42. Obviously.

“To Everything? To the great Question of Life, the Universe and everything?”
Both of the men had been trained for this moment, their lives had been a preparation for it, they had been selected at birth as those who would witness the answer, but even so they found themselves gasping and squirming like excited children.
“And you’re ready to give it to us?” urged Loonquawl.
“I am.”
“Now,” said Deep Thought.
They both licked their dry lips.
“Though I don’t think,” added Deep Thought. “that you’re going to like it.”
“Doesn’t matter!” said Phouchg. “We must know it! Now!”
“Now?” inquired Deep Thought.
“Yes! Now…”
“All right,” said the computer, and settled into silence again. The two men fidgeted. The tension was unbearable.
“You’re really not going to like it,” observed Deep Thought.
“Tell us!”
“All right,” said Deep Thought. “The Answer to the Great Question…”
“Of Life, the Universe and Everything…” said Deep Thought.
“Is…” said Deep Thought, and paused.
“Forty-two,” said Deep Thought, with infinite majesty and calm. 

– Douglas Adams: The Hitchhikers Guide to The Galaxy.

42 is the ASCII code for an asterisk (*), often known in computer programming as a “wildcard”. Wildcards are used as a programming shorthand for “anything”, for example you could use “*@pictureengine.co.uk” to mean any Picture Engine email address. Adams knew this, being a massive science nerd, and was therefore telling us in computer language that the meaning of life is

“anything you want it to be”.

You’re welcome…