Say, for 32-bit OS, we say that the largest available integer that the CPU can handle natively is 4,294,967,295, which in "human language" is 4 Gigabytes.
So for a 64-bit CPU, such number is 18,446,744,073,709,551,615, but how would you call it? A gazillion?
The first number is called 4 billion 294 million 967 thousand 295. Likewise, the larger number is 18 quintillion 446 quadrillion 744 trillion 73 billion 709 million 551 thousand 615 (given in short scale, so not everyone agrees that the first number are "billions" – they'll say it's "milliards").
4 gigabytes is the addressable memory range in 32 bit machines (simplified). The term 4 gigabytes is from the SI prefix for billion and just means "4 billion bytes" (similar to 1 kilometer = 1000 meters). So you could call that number "4 giga" if you want to confuse everyone you're talking to.
It's a bit more than 4 billion since we're actually dealing with powers of two rather than powers of 10, so the "kilo" is 1024, not 1000. Alternative binary prefixes exist, so you'd more correctly call this number 4 gibibytes.
Likewise, the larger number, used as a prefix, would be 16 exa or 16 exbi.
