Remedy is not just relief, it is the means of obtaining relief.
“The ideas of right and remedy are inseparable. ‘Want of right and want of remedy are the same thing.’” (Edwards v. Kearzey (1877) 96 U.S. 595, 600) “‘Remedy’ is a more extensive term and refers to the method by which the action is effectuated. (Frost v. Witter, 132 Cal. 421.) ‘Remedy’ is not redressed or relief, but is the means by which a wrong is redressed and relief obtained.” (Painter v. Berglund (1939) 31 Cal.App.2d 63, 70)
There are many vehicles to obtain relief. They come in different names, but no matter what, one thing matters most.
“Call it a motion for a new trial, arrest of judgment, mandamus, prohibition, coram nobis, coram vobis, audita querela, certiorari, capias, habeas corpus, ejectment, quare impedit, bill of review, writ of error, or an application for a Get-Out-of-Jail Card; the name makes no difference. It is substance that controls.” (Melton v. United States, (7th Cir. 2004) 359 F.3d 855, 857)
The Core Remedies
There are a number of remedies, which is to say the method of obtaining relief. Which is the correct to chose depends on what stage you are at in a case and what you want to achieve.
The classic remedy in the United States for people in custody, be it in prison, jail, on probation or parol is called habeas corpus. It is a terribly complex body of law for how simple it was intended to be. If you believe and can prove that your conviction was obtained in violation of the constitution then give habeas corpus a good look. If you are not represented then you must use form HC-001 if you are in California.
If you are no longer in custody or no longer on probation or no longer on parol, then you cannot use habeas corpus. Depending on what you are trying to accomplish, the correct remedy is something called a petition for a writ of mandamus (writ of mandate). This is used to compel a person or court to do their job. If your conviction was illegal then the government has an obligation to not let it continue.
There are also statutory remedies for persons no longer in custody. Such as Pen. Code sec. 745, (racist sentencing or conviction); sec. 1473.6 (newly discovered evidence of fraud or lies); sec. 1473.7 (newly discovered evidence proving innocence, not warned of immigration consequences when convicted).
If you suffer from a void judgment then you can make a non-statutory motion to vacate the judgment if no longer in custody. But that is only for void, not voidable. Void means a complete lack of authority over the parties or subject. (Legally called lack of subject matter jurisdiction or fundamental jurisdiction.) Voidable means acted contrary to law, or not according to statutory procedure. (Legally called acting in excess of jurisdiction.)