An only child.
Alone in a category.
without any others being included or involved
"was entirely to blame"
"a school devoted entirely to the needs of problem children"
"he works for Mr. Smith exclusively"
"did it solely for money"
"the burden of proof rests on the prosecution alone"
"a privilege granted only to him"
with nevertheless the final result
"He arrived only to find his wife dead"
"We won only to lose again in the next round"
Without others or anything further; exclusively.
Under the condition that; but.