inaugural speech; inaugural address
Refers to a formal speech given upon assuming an office, especially by a political leader.
新大統領の就任演説は国民に希望を与えた。
The new president's inaugural speech gave hope to the nation.
就任演説 specifically means a formal inaugural speech given at a ceremony, while 第一声 can refer more loosely to the first public remarks a new leader makes.
Compound of 就任 (assumption of office) and 演説 (speech).