company

1. a corporation — or, less commonly, an association, partnership, or union — that carries on a commercial or industrial enterprise

2. a corporation, partnership, association, joint-stock company, trust, fund, or organized group of persons, whether incorporated or not, and (in an official capacity) any receiver, trustee in bankruptcy, or similar official, or liquidating agent, for any of the foregoing

[Bryan A. Garner, Black’s Law Dictionary, 8^th Edition, West Group (2004).]

corporation

an entity (usu. a business) having authority under law to act as a single person distinct from the shareholders who own it and having rights to issue stock and exist indefinitely; a group or succession of persons established in accordance with legal rules into a legal or juristic person that has legal personality distinct from the natural persons who make it up, exists indefinitely apart from them, and has the legal powers that its constitution gives it

[Bryan A. Garner, Black’s Law Dictionary, 8^th Edition, West Group (2004).]

an invisible, intangible, artificial creation of the law existing as a voluntary chartered association of individuals that has most of the rights and duties of natural persons but with perpetual existence and limited liability

[Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated.]

firm

1. the title under which one or more persons conduct business jointly

2. the association by which persons are united for business purposes. Traditionally, this term has referred to a partnership, as opposed to a company. But today it frequently refers to a company.

[Bryan A. Garner, Black’s Law Dictionary, 8^th Edition, West Group (2004).]

1: the name or title under which a company transacts business

2: a partnership of two or more persons that is not recognized as a legal person distinct from the members composing it

3: a business unit or enterprise

[Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated.]

concern

a commercial or industrial enterprise and the people who constitute it

宋雷老师认为，company、corporation和firm三词均有“公司”之义。鉴于在英国公司法被称为company law，因此在英国，company常用作指已经登记注册的正式公司，包括大公司在内。而在美国，公司法被称为corporation law或corporate law，因而美国人多视corporation为按公司法正式注册登记成立的大公司，而视company为规模较小或不甚正式的公司，故我们对company和corporation的理解应依照英、美两国不同的国情而定。以往，firm多指合伙企业或未按公司法规定的程序而正式成立的实体或企业，其多不是具有完全法人资格的独立实体，这在破产法上尤为重要，尽管现在其也常被用于指company。

在我看来，排除宋雷老师所说的英美两国的理解差异，在美国，corporation多指按公司法登记注册成立的股份有限公司；而company一词应用更为广泛，除可表示依公司法登记注册成立的股份有限公司外，还可表示其它的合伙企业、非法人企业等。Firm原来常用于指合伙企业，现在也常用于指company。Concern则常常用于指个人的、家族式的小企业或商号。