extra

additional, extra, supplementary

名词 extra:

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

形容词 extra:

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

副词 extra:

1. unusually or exceptionally

1. Luggage in excess of 100 kg will be charged extra.
   超过100公斤的行李要额外收费。
2. I'm so envious of you getting an extra day's holiday.
   我真羡慕你得到一天额外的假期。
3. The teacher disciplined the class by giving them extra homework.
   那教师布置额外的家庭作业来处罚学生。
4. He paid the extra cost without a murmur.
   他付了额外的费用而毫无怨言。
5. This dress is of extra fine quality.
   这件衣服的质地特别好。
6. There were so many people that the company put on extra buses.
   人数太多，公司加开了公共汽车。

ex.tra

adj.Abbr. ext.,ex.,X（形容词）缩写 ext.,ex.,X
More than or beyond what is usual, normal, expected, or necessary.See Synonyms at superfluous
超额的，额外的：多于或超过通常的、正常的、意料的或必要的参见 superfluous
Better than ordinary; superior:
特佳的：比正常的好；优秀的：
例句：
extra fineness.
特别好

Subject to an additional charge:
另收费的：须经额外加费的：
例句：
a pizza with extra cheese.
奶酪要另行收费的比萨饼

n.（名词）
Something more than is usual or necessary.
额外物：超出平常的或必要的东西
Something, such as an accessory on a motor vehicle, for which an additional charge is made.
附加物：须另外收费的某物，如机动车上的附件
A special edition of a newspaper.
号外：报纸中特殊的一版

An additional or alternate worker.
额外的人手
A performer hired to play a minor part, as in a crowd scene.
临时演员：如拍摄电影群众场面时雇来的临时演员
Something of exceptional quality.
上品，特级品：质量特别好的东西
adv.（副词）
To an exceptional extent or degree; unusually:
格外地，特别地：达到例外的程度或限度；格外地：
例句：
extra dry.
格外干

来源：
Probably short for extraordinary
可能为 extraordinary的简写