It's a Card - for the reasons Ceti cites on page one.
The Master System plays cards and cartridges, not cartridges and cartridges.
The strict definition argument has some merit, but there are enough media variations in the world of video games to warrant specific terminology. Just like cars and trucks are both automobiles, CDs, Turbochips, and game packs are all cartridges.
I wonder why they called it a turbochip in the manual.. to make it sound more fun?
To make it sound too flippin' extreme.
"Turbochip" is the kind of thing one plays after a day of riding the wave, shootin' the curl, and going totally aggro on the halfpipe.