a

• n. 字母A；第一流的；学业成绩达最高标准的评价符号
• abbr. [物]安（ampere）

不定冠词an在发展过程中出现的缩略形式，应用于首字母为辅音的单词及h发音单词。

a

a: [OE] The indefinite article in English is ultimately identical with the word one (as is the case, even more obviously, in other European languages – French un, German ein, and so on). The ancestor of both a(n) and one was ān, with a long vowel, but in the Old English period it was chiefly used for the numeral; where we would use a(n), the Anglo-Saxons tended not to use an article at all. Ān begins to emerge as the indefinite article in the middle of the 12th century, and it was not long before, in that relatively unemphatic linguistic environment, its vowel became weakened and shortened, giving an.

And at about the same time the distinction between an and a began to develop, although this was a slow process; until 1300 an was still often used before consonants, and right up to 1600 and beyond it was common before all words beginning with h, such as house.
=> one

a (1)

indefinite article, mid-12c., a variation of Old English an (see an) in which the -n- began to disappear before consonants, a process mostly complete by mid-14c. The -n- also was retained before words beginning with a sounded -h- until c. 1600; it still is retained by many writers before unaccented syllables in h- or (e)u-, but is now no longer normally spoken as such. The -n- also lingered (especially in southern England dialect) before -w- and -y- through 15c.

a (2)

as in twice a day, etc., from Old English an "on," in this case "on each." The sense was extended from time to measure, price, place, etc. The habit of tacking a onto a gerund (as in a-hunting we will go) died out 18c.

1. Liver and kidney are particularly rich in vitamin A.

肝脏和肾脏富含维生素A。

来自柯林斯例句

2. Details are available from the Hon. Sec. A.R. Bushby.

详情可咨询名誉秘书A.R.布什比。

来自柯林斯例句

3. I was assigned to Troop A of the 10th Cavalry.

我被派往第10骑兵队的A连。

来自柯林斯例句

4. Passes are graded from "A" down to "E".

及格分为A到E几等。

来自柯林斯例句

5. Use only clean, Grade A, perfect eggs.

只用表面干净、无磕碰的A级鸡蛋。

来自柯林斯例句