What is a Bitcoin Private Key?

[iamchinito]

The private key is used to generate the public key through an irreversible process.
In the case of Bitcoin the Private key is turned into the Public key through Elliptic Curve Cryptography or ECC for short.
If you lose your private key you can't access your Bitcoins, and if someone else finds it out they can steal them.

A Private key is just a long number and in the case of a Bitcoin it is a number between 1 & 1.158x 10^77.