Unaccompanied asylum seeking child
The definition for immigration purposes of an unaccompanied asylum seeking children is given by the UKBA as "a person under 18 years of age or who, in the absence of documentary evidence establishing age, appears to be under that age" who "is applying for asylum in their own right; and is separated from both parents and not being cared for by an adult who by law or custom has responsibility to do so" [1]
The UKBA does not consider a child to be unaccompanied where there is an adult prepared to take responsibility for them, although they will involve the local authorities where there is a concern about the child's relationship with the 'responsible' adult.
[1] See BIA Guidance on Processing Asylum Applications from Children