An IOC-defined designator
The codes were first introduced unofficially in the daily program for the 1952 Summer Olympics in Helsinki. They are not always three-letter codes; some consist of four letters like USSR or CHIN, or two letters like SF for Finland, or include hyphens like G-B or T-S. It wasn't until the Rome Games in 1960 that the use of codes became systematic, where they were made up of the first three letters of the country's name in French or Italian (e.g. GIA for Japan). However, it wasn't until the Montreal Games that the codes were officially standardized.
Here is the list of current country codes used by the IOC with their corresponding denominations:
ALG Algeria
ANG Angola
ANT Antigua and Barbuda
ARG Argentina
ARU Aruba
ASA American Samoa
AUS Australia
AUT Austria
BEL Belgium
BER Bermuda
BRA Brazil
CAN Canada
CAY Cayman Islands
CHI Chile
CHN China
CIV Ivory Coast
CMR Cameroon
COL Colombia
CRO Croatia
CYP Cyprus
CZE Czech Republic
DEN Denmark
EGY Egypt
ESA Salvador
ESP Spain
EST Estonia
FIJ Fiji
FIN Finland
FRA France
GBR Great Britain
GRE Greece
GUA Guatemala
HKG Hong Kong
HUN Hungary
IND India
IRL Ireland
ISR Israel
ITA Italy
IVB British Virgin Islands
JPN Japan
KOR South Korea
KUW Kuwait
LCA Sainte-Lucie
LTU Lithuania
MAS Malaysia
MEX Mexico
MNE Montenegro
MOZ Mozambique
MRI Maurice
NED Netherlands
NOR Norway
NZL New Zealand
PER Peru
POL Poland
POR Portugal
PUR Puerto Rico
ROU Romania
SAM Samoa
SGP Singapore
SLO Slovenia
SUI Switzerland
SVK Slovakia
SWE Sweden
THA Thailand
TUR Turkey
URU Uruguay
USA United States
