a metric unit of length equal to one ten billionth of a meter (or 0.0001 micron); used to specify wavelengths of electromagnetic radiation
同义词：angstrom, angstrom unit
any of several fat-soluble vitamins essential for normal vision; prevents night blindness or inflammation or dryness of the eyes
同义词：vitamin A, antiophthalmic factor, axerophthol
one of the four nucleotides used in building DNA; all four nucleotides have a common phosphate group and a sugar (ribose)
同义词：deoxyadenosine monophosphate
(biochemistry) purine base found in DNA and RNA; pairs with thymine in DNA and with uracil in RNA
同义词：adenine
the basic unit of electric current adopted under the Systeme International d'Unites
同义词：ampere, amp
the 1st letter of the Roman alphabet
同义词：letter a
the blood group whose red cells carry the A antigen
同义词：type A, group A
I have engaged a room at this hotel.
我已经在这个旅馆里预订了一个房间。
You should give it a try.
你应该试一试。
I don't care what a car looks like as long as it gets me from A to B.
我倒不在乎汽车的样子，只要能把我从一处载到另一处就行了。
The function of an adjective is to describe or add to the meaning of a noun.
形容词的作用是描述或增加名词的意思。
China is a developing country.
中国是一个发展中国家。
You're just a good for nothing bum!
你一无是处！
Declension means varying the forms of nouns, pronouns and adjectives in a sentence.
变格是指名词、代词和形容词在句中的形式变化。
Would you do me a favor?
你能帮我一个忙吗？
a 或 A
n.（名词）
【复数】 a`s 或 A`s
The first letter of the modern English alphabet.
A字母：现代英语字母表中第一个字母
Any of the speech sounds represented by the lettera.
A的发音：a 所表示的读音
The first in a series.
序列中的第一
The best or highest in quality or rank:
甲等，甲级：在质量、等级上最好或最高：
例句：
grade A milk.
优质牛奶
Something shaped like the letter A.
A字形：形状象字母A的物体
Music
【音乐】
The sixth tone in the scale of C major or the first tone in the relative minor scale.
A音或A调：C大调音阶中的第六个音或C小调音阶中的第一个音
A key or scale in which A is the tonic.
A调音阶：以A调为主音调的音键或音阶
A written or printed note representing this tone.
A调的音符：代表A调的手写体或印刷体的音符
A string, key, or pipe tuned to the pitch of this tone.
发A调的乐器：调至A调音高的弦乐器、琴键或管乐器
A One of four types of blood in the ABO system.
A A型血：ABO系统中四种血型的一种
a
a 2
indef.art.（不定冠词）
Used before nouns and noun phrases that denote a single but unspecified person or thing:
非特指的一个：用在名词或名词短语前，表示单数及不确定的人或物：
例句：
a region; a person.
一个地区，一个人
Used before terms, such asfew or many, that denote number, amount, quantity, or degree:
用在如few 或 many 之前，表示总和、总数、数量或程度：
例句：
only a few of the voters; a bit more rest; a little excited.
只有少许投票者；稍微多休息会儿；有些激动
Used before a proper name to denote a type or a member of a class:
用在一专有名称前来表示一类中的一种或一员：
例句：
the wisdom of a Socrates.
苏格拉底式的智慧
Used before a mass noun to indicate a single type or example:
用于物质名词前表示单一种类或事例：
例句：
a dry wine.
无甜味葡萄酒
The same:
同样，同类：
例句：
birds of a feather.
同一种类的鸟
Any:
任何一个：
例句：
not a drop to drink.
滴水未沾
来源：
Middle English
中古英语
variant of an [an] * see an
an的变体 [不定冠词an] *参见 an
In modern written English, the forma is used before a word beginning with a consonant sound, however it may be spelled (a frog, a university, a euphemism ). The forman is used before a word beginning with a vowel sound ( an orange, an hour ). At one time,an was an acceptable alternative before words beginning with a consonant sound but spelled with a vowel ( an one, an united appeal ), but this usage is now entirely obsolete. ·An was also once a common variant before words beginning with h in which the first syllable was unstressed; thus 8th-century authors wrote eithera historical or an historical but a history, not an history. This usage had a phonetic justification inasmuch as the initialh in words such as historical and heroic was often dropped, a practice that was largely abandoned by the late 9th century in educated speech.By 92' H.W. Fowler could regard the continued use ofan before such words as pedantic. Nowadays it survives primarily before the wordhistorical; one rarely encounters a reference toan hysterectomy or an hereditary trait. The use of a phrase likean historic opportunity is a harmless adornment in formal writing.
在现代书面英语中a 的形式用于以辅音开头的单词前， 但这个词可能被拼成（a frog，a university，a euphemism ）。 an 的形式用于以元音开头的词前（ an orange，an hour ）。 有一度an 用于以辅音开头但拼写形式为元音的单词前的形式也被认可（ an one，an united appeal ）， 但这种用法现在已被废弃了。An 曾是用于第一个音节不重读，且以 h 为开头的单词前的一种常见变体形式； 所以8世纪的作家既可用a historical 也可用 an historical， 但不可用 a history 也不可用 an history。 当h 在这样一些单词，如 historical 和 heroic 中作为起首字母而又不发音时，这种用法在语音学上是合理的。 这种用法在9世纪末的高雅用语中已基本废弃了。92'年时H.W.福勒还认为an 继续在这些单词前使用太具学究气了。 现在这种用法主要在historical 前时还保存使用； 在an hysterectomy 或 an hereditary trait 之前则极为少见。 在正式的书面语中，象an historic opportunity 这种用法也不失为一种装饰
a
a 3
prep.（介词）
In every; to each; per:
单一；每一；每：
例句：
once a month; one dollar a pound.
每月一次；一磅一美元
来源：
Middle English
中古英语
from Old English an [in] * see on
源自 古英语 an [在…里] *参见 on
a
a 4
v.aux.Informal （助动词）【非正式用语】
Have:
相当于助动词have：
例句：
He`d a come if he could.
如果他能来的话，他早就来了
来源：
Middle English
中古英语
alteration of haven [to have] * see have
haven的变化 [有] *参见 have
a
a 5
abbr.（略语）
Also a. Absent.
也作 a. Absent.
Physics Acceleration.
【物理学】 Acceleration.
Also a. Are [measurement]
也作 a. Are[测量]
