Plaintiff （someone who makes a legal complaint against someone else in court）：

原告，为通用词，常用于普通法法院的一般民事或刑事案件的初审中，与其相对的被告称为defendant。

示例（某民事起诉状中的请求）：

Our claim is that the court can order the defendant Shenyang Medical Equipment Company immediately pay to the plaintiff the contract sum o f USD 70 million and interest (From May 20, 2010 to the date of actual payment, it shall be calculated according to the Loan Prime Rate published by the National Interbank Lending Center in the same period.

我司请求判令被告沈阳医疗器械有限公司立即向原告支付所欠付的合同款7千万元人民币及利息（自2010年5月20日起至实际给付之日止，按照同期全国银行间同业拆借中心公布的贷款市场报价利率计算。

Appellant （An appellant is someone who is appealing a court's decision after they have been judged guilty of a crime. ）

“上诉人”，可用于刑事或民事上诉案件中，与其相对的“被上诉人”或“应诉人”称为respondent或appellee。

示例：

If either the appellant or the respondent objected to the appointment of the replacement board member, another mutually agreed hearing date would be fixed so as to allow the return of the absent board member to attend the hearing. 

倘上诉人或应诉人对替代委员会委员的委任提出异议， 上诉委员团会另订一个双方协定的聆讯日期，以便该名缺勤的委员会委员回来出席聆讯。

Applicant用于申请某一特别命令的诉讼，也可译为“申请人”，多用于国际诉讼中，与其相对的“被申请人”或“被告”常被称为respondent。

示例（来自某财产保全申请书）：

In order to prevent the respondent Alexander from transferring property, the applicant hereby applies for the property attachment.

为防止被申请人Alexander转移财产，申请人特此申请财产保全。

Petitioner （A petitioner is a person who brings a legal case to a court of law.）

原告或申请人，多用于衡平法诉讼中，如衡平法院、大法官法院的诉讼以及家事法院的离婚案件，遗嘱验证法院的遗嘱验证案等，与之相对的被告或被申请人常被称为respondent。

示例：

References to the rights of the petitioner and the respondent to be represented by lawyers during a trial of an election petition are omitted. 

有关呈请人及答辩人有权在选举呈请审讯期间以律师为其代表的提述均予省略。 

此外，还有一个常见的可以表示原告的词，claimant，它常出现在英国民事诉讼中，与之相对的被告被称为defendant。