| XAF | BEAC (Banque Des ?tats De L'Afrique Centrale) Central Africa CFA (Coop?ration Financi?re En Afrique Centrale) Franc (ISO 4217 Currency Code) | - | Astronomy |
| XAG | Silver Troy Ounce (ISO 4217 Currency Code) | - | Astronomy |
| XAU | Gold Troy Ounce (ISO 4217 Currency Code) | - | Astronomy |
| XBA | European Composite Unit (EURCO; Bonds Market Unit) (ISO 4217 Currency Code) | - | Astronomy |
| XBB | European Monetary Unit (E.M.U.; Bonds Market Unit) (ISO 4217 Currency Code) | - | Astronomy |
| XBML | EXtensible Battle Management Language | - | Astronomy |
| XC | Ninety (in Roman Numerals) | - | Astronomy |
| XCBL | XML Common Business Library | - | Astronomy |
| XCD | East Caribbean Dollar (ISO 4217 Currency Code) | - | Astronomy |
| XE | Xenon | - | Astronomy |
| XH | Xhosa Language (ISO 639-1 Code) | - | Astronomy |
| XHO | Xhosa Language (ISO 639-2 Code) | - | Astronomy |
| XHR | EXtra High Image Resolution - XMLHTTPRequest | - | Astronomy |
| XHTML | EXtensible Hypertext Markup Language | - | Astronomy |
| XI | Eleven (in Roman Numerals) | - | Astronomy |
| XII | Twelve (in Roman Numerals) | - | Astronomy |
| XIII | Thirteen (in Roman Numerals) | - | Astronomy |
| XIV | Fourteen (in Roman Numerals) | - | Astronomy |
| XIX | Nineteen (in Roman Numerals) | - | Astronomy |
| XL | EXtra Large -Forty (in Roman Numerals) | - | Astronomy |
| XML | EXtensible Markup Language | - | Astronomy |
| XMSF | EXtensible Modelling And Simulation Framework | - | Astronomy |
| XO | ???†‡?…??? ????‰???? (Xristianos Orthodoksos | - | Astronomy |
| XOF | BCÂAO (Banque Centrale Des ?tats De L'Afrique De L'Ouest) West African CFA (Communaut? Financi?re D'Afrique) Franc (ISO 4217 Currency Code) | - | Astronomy |
| XP | EXperience Point (role-playing Games) | - | Astronomy |
| XPD | Palladium Troy Ounce (ISO 4217 Currency Code) | - | Astronomy |
| XPF | CFP Franc (ISO 4217 Currency Code) | - | Astronomy |
| XPT | Platinum Troy Ounce (ISO 4217 Currency Code) | - | Astronomy |
| XRML | EXtensible Rights Markup Language | - | Astronomy |
| XS | Xiaolin Showdown | - | Astronomy |
| XSL | EXtensible Style Language | - | Astronomy |
| XUL | XML User Interface Language | - | Astronomy |
| XV | Fifteen (in Roman Numerals) | - | Astronomy |
| XVI | Sixteen (in Roman Numerals) | - | Astronomy |
| XVII | Seventeen (in Roman Numerals) | - | Astronomy |
| XVIII | Eighteen (in Roman Numerals) | - | Astronomy |
| XX | Twenty (in Roman Numerals) | - | Astronomy |
| XXX | Hardcore Pornography (film Rating) - Thirty (in Roman Numerals) | - | Astronomy |
