In legal terminology, a complaint is a formal legal document that sets out the facts and legal reasons (see: cause of action) that the filing party or parties (the plaintiff[s] beliefs are sufficient to support a claim against the party or parties against whom the claim is brought [the defendant(s) that entitle(s) the plaintiff(s) to a remedy (either money damages or injunctive relief)]). For example, the Federal Rules of Civil Procedure (FRCP) that govern civil litigation in United States courts provide that a civil action is commenced with the filing or service of a pleading called a complaint. Civil court rules in states that have incorporated the Federal Rules of Civil Procedure use the same term for the same pleading.

In some jurisdictions, specific types of criminal cases may also be commenced by the filing of a complaint, also sometimes called a criminal complaint or felony complaint. All criminal cases are prosecuted in the name of the governmental authority that promulgates criminal statutes and enforces the police power of the state with the goal of seeking criminal sanctions, such as the State (also sometimes called the People) or Crown (in Commonwealth realms). In the United States, the complaint is often associated with misdemeanor criminal charges presented by the prosecutor without the grand jury process. In most U.S. jurisdictions, the charging instrument presented to and authorized by a grand jury is referred to as an indictment.

Complaints', in general usage, are expressions of displeasure.

Sourced

• Every one must see daily instances of people who complain from a mere habit of complaining.
  • Richard Graves, Olla Podrida 6 October 1787

• There is an unfortunate disposition in a man to attend much more to the faults of his companions which offend him, than to their perfections which please him.
  • Fulke Greville, 1st Baron Brooke

• I have noticed this, that when a man is full of the Holy Ghost he is the very last man to be complaining of other people.
  • Dwight L. Moody, reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895), p. 420.

• I pity the man who can travel from Dan to Beersheba and cry, 'Tis all barren!
  • Laurence Sterne, A Sentimental Journey Through France and Italy (1768) In the Street, Calais

• Some people are never content with their lot, let what will happen. Clouds and darkness are over their heads, alike whether it rain or shine. To them every incident is an accident, and every accident a calamity.
  • , reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895), p. 420.

• From mad dogs and grumbling professors may we all be delivered, and may we never take the complaint from either of them.
  • , reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895), p. 421.

• Nothing is easier than fault-finding. No talent, no self-denial, no brains, no character, is required to set up in the grumbling business. But those that are moved by a genuine desire to do good have little time for murmuring or complaint.
  • Robert West, reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895), p. 420.