This is the 13590th most frequent Russian word.
невиновную
"Innocent" (feminine form in accusative case).
Here, "невиновную" describes the girl as innocent and is used as an accusative feminine singular adjective.
Он пытался защитить невиновную девушку.
He tried to protect the innocent girl.
In this case, "невиновную" is used as an accusative form of the adjective modifying an implied feminine noun, meaning the person acquitted is female and declared innocent.
Суд оправдал невиновную.
The court acquitted the innocent woman.
Here, "невиновной" is declined in the dative case, referring to helping a woman who is innocent.