This is the 496th most frequent Lithuanian word.
vienintelis
The Lithuanian word "vienintelis" means "only" or "sole" in English.
In this sentence, "vienintelis" is used as an adjective meaning "only" to emphasize that there is no other friend but this one.
Jis yra vienintelis mano draugas.
He is my only friend.
Here, "vienintelė" modifies the noun "galimybė" to indicate it was a singular and unique opportunity.
Tai buvo vienintelė galimybė.
That was the only opportunity.
In this instance, "vienintelė" is used to specify that she desires to be the sole person possessing the understanding.