Yes, it's a word, not an acronym. Exa (pronounced EKS-ah) is the Greek prefix for 10^18 or 1 with 18 zeroes.
other familiar prefixes are kilo, mega, and giga. After giga,
the next prefixes that will be used will be tera, peta and then exa
exa is from the same word as Hex, meaning 6 - a hexagon has 6 sides
exa essentially means that there are 6 sets of zeroes after the number.
If you've used computers for a while, you remember when memory was measured in kilobytes and hard drives in megabytes. Now several gigabytes is common for memory and several hundred gigabytes is common for desktop hard drives. Before Google Maps was released, Microsoft had a website called terraserver with satellite images available. At the time it was a big deal to have a terabyte database.
Soon, a terabyte hard drive will be available, and you'll hear about petabyte databases.