Complaint, n.
Etymology: [F. complainte. See Complain.]
1. Expression of grief, regret, pain, censure, or resentment; lamentation; murmuring; accusation; fault-finding. I poured out my complaint before him. Ps. cxlii. 2. Grievous complaints of you. Shak.
2. Cause or subject of complaint or murmuring. The poverty of the clergy in England hath been the complaint of all who wish well to the church. Swift.
3. An ailment or disease of the body. One in a complaint of his bowels. Arbuthnot.
4. (Law)
2 more senses
A formal allegation or charge against a party made or presented to the appropriate court or officer, as for a wrong done or a crime committed (in the latter case, generally under oath); an information; accusation; the initial bill in proceedings in equity.
Syn. -- Lamentation; murmuring; sorrow; grief; disease; illness; disorder; malady; ailment.
Shades of meaning
Fernald has no entry of its own for complaint; he lists it under disease.
Where he writes about it:
Complain has a formal and legal meaning, which the other words have not, signifying to make a formal accusation, present a specific charge; the same is true of the noun complaint. — from complain
Complaint is a popular term, which may be applied to any degree of ill health, slight or severe. — from disease