There is a difference between specific gravity, original gravity, and final gravity.
Here is just a Specific Gravity notation pet-peeve of mine:
SG – Specific Gravity;
This abbreviation is used to refer to any time a gravity reading is taken but it does not denote a specific time or occasion that it was taken.
Example: “I took an SG reading during my boil.”
OG – Original Gravity/Stating Gravity;
This abbreviation refers to the gravity reading you took prior to the yeast getting started. Normally, this reading is taken at the fermentor before the yeast is pitched.
Example: “My wort was 1.045 OG before I pitched my yeast.”
FG – Final Gravity;
This abbreviation refers to the gravity reading taken once fermentation is completed. It is where the wort/beer finally came to rest after the fermentation was complete. Moreover, it’s the gravity of the beer as it is presented to the consumer.
Example: “This beer is fairly dry with an FG of 1.008.”
All these abbreviations are SG readings. We use OG and FG to designate specific times that they were taken as bench marks for calculating things like ABV and describing flavors like dryness or residual sweetness.
OK, pet-peeve closed.