Capital murder was a statutory offence of aggravated murder in Great Britain and Northern Ireland and the Republic of Ireland. In the United States, a capital murder is any murder that makes the perpetrator eligible for the death penalty. 
In Great Britain, this offence was created by section 5 of the Homicide Act 1957. Previously all murders carried the death penalty on conviction, but the 1957 Act limited the death penalty to the following cases:
In all other cases murder carried the mandatory penalty of imprisonment for life.
Section 1 of the Murder (Abolition of Death Penalty) Act 1965 abolished the separate category of capital murder, and all murders now carry the mandatory penalty of imprisonment for life.
In Northern Ireland, this offence was created by section 10 of the Criminal Justice Act (Northern Ireland) 1966.
On the trial of an indictment for capital murder, the jury could not return an alternative verdict to the offence charged in that indictment under section 6(2) of the Criminal Law Act (Northern Ireland) 1967.
Sections 1(4) and (5) of the Northern Ireland (Emergency Provisions) Act 1973 read:
(4) For the purpose of any proceedings on or subsequent to a person's trial on a charge of capital murder, that charge and any plea or finding of guilty of capital murder shall be treated as being or having been a charge, or a plea or finding of guilty, of murder only; and if at the commencement of this Act a person is under a sentence of death for capital murder, the sentence shall have effect as a sentence of imprisonment for life.
(5) In this section "capital murder" means a murder which immediately before the commencement of this Act is a capital murder within the meaning of section 10 of the Criminal Justice Act (Northern Ireland) 1966.
The term "capital murder" is used in only a handful of U.S. states; however, 31 states and United States federal government currently allow for capital punishment, and each has its own terminology for an offense punishable by death. In most states, the term "First-Degree Murder" is used; others may use the term "Aggravated Murder" (such as New York, Ohio, Oregon, Utah, and Vermont), and some use simply "Murder". Only seven states use the term "Capital Murder" (such as Alabama, Arkansas, Kansas, Mississippi, New Hampshire, Texas, and Virginia), and only the state of Georgia uses the term "Malice Murder".
Terminology is varied, however, and not all offenses are parallel between the states. In some, first-degree murder is a very broad term defined by a number of circumstances, only a few of which make a defendant eligible for execution. In other jurisdictions, an offense carrying the death penalty is strictly defined and is separate from other, similar crimes.
Although legal definitions vary, capital murder in the United States usually means murder involving one or more of the following factors:
Some states may include other factors which amount to capital murder or its legal equivalent.
Capital offenses in the United States are not punishable by death exclusively. Most states afford courts the option of imposing either the death penalty or a life sentence upon conviction, though lesser sentences are rare and in some cases legally impossible. Depending on the state, the presiding judge may determine the sentence, or the decision may be left to the jury.
The United States Supreme Court has placed limitations on the use of the death penalty and has prohibited its use in cases where the offender is mentally incompetent, or was under the age of 18 at the time of the offense.