For those familiar with shotguns, its a bore diameter. The smaller the number, actually, the larger the hole. So a 22 gauge shotgun has a much smaller bore diameter, or hole, than a 12 gauge shotgun.
Same with a needle.
*Intermission for funny story from the ER*
To start an IV, nurses usually use a 24 gauge needle or smaller. Unlike drawing blood, which is bearable, having an IV started is painful. I know because we had to practice on each other in PA school.
So there was this nurse I knew at OU Presby's ER, who, if she ran into a teenager that had done something especially stupid; and was simultaneously disrespecting their parents; or the nursing staff; and then needed intravenous medication to be rescued-
Would always use an 18 gauge needle to start their IV.
"Don't be nobody in their right mind do something like that to their own mother," she would mutter as she searched for the big needle.
The pain from a 24 gauge needle to start an IV is significant. An 18 is torture.
*Back to the real story*
My needles for spinal taps are anywhere from one to three and a half inches long. Their gauge isn't too bad- usually about a 20.
But a bone marrow biopsy needle is an 8. And, its five inches long.