Yechezkel ben Yehuda HaLevi Landau was an influential authority in halakha. He is best known for the work Noda Biyhudah, by which title he is also known.
Yechezkel Landau
In Jewish law, a posek is a legal scholar who determines the application of halakha, the Jewish religious laws derived from the written and Oral Torah in cases of Jewish law where previous authorities are inconclusive, or in those situations where no clear halakhic precedent exists.
Toledot HaPoskim, History of the Jewish Codes, by Chaim Tchernowitz.