testimony / witness / declaration of faith
Feminine verbal noun (maṣdar) of شَهِدَ (to witness/testify). Related: شَاهِدٌ (witness), شَهِيدٌ (martyr/witness), مَشْهَدٌ (scene/gathering). Also means martyrdom.
The shahādah (لَا إِلَهَ إِلَّا اللَّهُ مُحَمَّدٌ رَسُولُ اللَّهِ) is the first pillar of Islam. Uttering it sincerely is what makes one a Muslim.