Collins COBUILD Advanced Learner’s Dictionary

2 ADJ An acid substance contains acid. □  These shrubs must have an acid, lime-free soil. ●  acid|ity /æs I d I ti/ N‑UNCOUNT □ [+ of ] …the acidity of rainwater.

3 N‑UNCOUNT The drug LSD is sometimes referred to as acid . [INFORMAL ]

4 → see also amino acid , hydrochloric acid , nitric acid , nucleic acid , sulphuric acid

a cid hou se N‑UNCOUNT Acid house is a type of electronic dance music with a strong, repeated rhythm.

acid|ic /əs I d I k/ ADJ Acidic substances contain acid. □  Dissolved carbon dioxide makes the water more acidic.

a cid rai n N‑UNCOUNT Acid rain is rain polluted by acid that has been released into the atmosphere from factories and other industrial processes. Acid rain is harmful to the environment.

a cid te st N‑SING The acid test of something is an important aspect or result that it might have, which allows you to decide whether it is true or successful. □ [+ of ] The acid test of a school is 'would you send your own children there?'

ac|knowl|edge ◆◇◇ /æknɒ l I dʒ/ (acknowledges , acknowledging , acknowledged )

1 VERB If you acknowledge a fact or a situation, you accept or admit that it is true or that it exists. [FORMAL ] □ [V that] We have to acknowledge that something is wrong with the system. □ [V n] Belatedly, the government has acknowledged the problem. □ [V -ed] There is an acknowledged risk of lung cancer from radon. [Also V n to-inf, V n as n/adj]

2 VERB If someone's achievements, status, or qualities are acknowledged , they are known about and recognized by a lot of people, or by a particular group of people. □ [be V -ed + as ] He is also acknowledged as an excellent goal-keeper. □ [V n] Some of the clergy refused to acknowledge the new king's legitimacy. [Also V n to-inf]

3 VERB If you acknowledge a message or letter, you write to the person who sent it in order to say that you have received it. □ [V n] The army sent me a postcard acknowledging my request.

4 VERB If you acknowledge someone, for example by moving your head or smiling, you show that you have seen and recognized them. □ [V n] He saw her but refused to even acknowledge her.

ac|knowl|edge|ment /æknɒ l I dʒmənt/ (acknowledgements ) also acknowledgment

1 N‑SING [oft N that] An acknowledgement is a statement or action which recognizes that something exists or is true. □  The President's resignation appears to be an acknowledgment that he has lost all hope of keeping the country together.

2 N‑PLURAL The acknowledgements in a book are the section in which the author thanks all the people who have helped him or her.

3 N‑UNCOUNT [oft a N ] A gesture of acknowledgement , such as a smile, shows someone that you have seen and recognized them. □  Farling smiled in acknowledgement and gave a bow.

4 N‑COUNT An acknowledgement is a letter or message that you receive from someone, telling you that something you have sent to them has arrived. □  I have received neither an acknowledgment nor a reply.

acme /æ kmi/ N‑SING The acme of something is its highest point of achievement or excellence. [FORMAL ] □ [+ of ] His work is considered the acme of cinematic art.

acne /æ kni/ N‑UNCOUNT If someone has acne , they have a skin condition which causes a lot of spots on their face and neck.

aco|lyte /æ kəla I t/ (acolytes ) N‑COUNT An acolyte is a follower or assistant of an important person. [FORMAL ] □  To his acolytes, he is known simply as 'the Boss'.

acorn /e I kɔː ^r n/ (acorns ) N‑COUNT An acorn is a pale oval nut that is the fruit of an oak tree.

acous|tic /əkuː st I k/ (acoustics )