additional, extra, supplementary

名詞 extra:

1. a minor actor in crowd scenes
   同義詞：supernumerary, spear carrier
2. an additional edition of a newspaper (usually to report a crisis)
3. something additional of the same kind
   同義詞：duplicate

形容詞 extra:

1. more than is needed, desired, or required
   同義詞：excess, redundant, spare, supererogatory, superfluous, supernumerary, surplus
2. added to a regular schedule
   同義詞：special
3. further or added
   同義詞：additional

副詞 extra:

1. unusually or exceptionally

1. Luggage in excess of 100 kg will be charged extra.
   超過100公斤的行李要額外收費。
2. I'm so envious of you getting an extra day's holiday.
   我真羨慕你得到一天額外的假期。
3. The teacher disciplined the class by giving them extra homework.
   那教師布置額外的家庭作業(yè)來處罰學(xué)生。
4. He paid the extra cost without a murmur.
   他付了額外的費用而毫無怨言。
5. This dress is of extra fine quality.
   這件衣服的質(zhì)地特別好。
6. There were so many people that the company put on extra buses.
   人數(shù)太多，公司加開了公共汽車。

ex.tra

adj.Abbr. ext.,ex.,X（形容詞）縮寫 ext.,ex.,X
More than or beyond what is usual, normal, expected, or necessary.See Synonyms at superfluous
超額的，額外的：多于或超過通常的、正常的、意料的或必要的參見 superfluous
Better than ordinary; superior:
特佳的：比正常的好；優(yōu)秀的：
例句：
extra fineness.
特別好

Subject to an additional charge:
另收費的：須經(jīng)額外加費的：
例句：
a pizza with extra cheese.
奶酪要另行收費的比薩餅

n.（名詞）
Something more than is usual or necessary.
額外物：超出平常的或必要的東西
Something, such as an accessory on a motor vehicle, for which an additional charge is made.
附加物：須另外收費的某物，如機(jī)動車上的附件
A special edition of a newspaper.
號外：報紙中特殊的一版

An additional or alternate worker.
額外的人手
A performer hired to play a minor part, as in a crowd scene.
臨時演員：如拍攝電影群眾場面時雇來的臨時演員
Something of exceptional quality.
上品，特級品：質(zhì)量特別好的東西
adv.（副詞）
To an exceptional extent or degree; unusually:
格外地，特別地：達(dá)到例外的程度或限度；格外地：
例句：
extra dry.
格外干

來源：
Probably short for extraordinary
可能為 extraordinary的簡寫