Karl Friedrich Geldner (17 December 1852 – 5 February 1929) was a German linguist best known for his analysis and synthesis of Avestan and Vedic Sanskrit texts.
The essay, which was in its expanded and published form titled Über die Metrik des jüngeren Avesta ("On the meter of the Younger Avesta"), was originally an answer to a prize essay question posed by the University of Tübingen's Faculty of Philosophy.
Although the theory was subsequently revised by others, Geldner's hypothesis was reinstated in 1983, and the lines of the Younger Avesta are today considered to be historically related to the Vedic meters of the gayatri family.
Although Geldner would have preferred to research the Vedas (he would later state to had "lost" 15 years working on the Avesta), following the publication of his doctoral thesis, Geldner began to work on a revision of Westergaard's edition of the Avesta.
Together with Richard Pischel he began to work on the Vedas, and their collaboration was subsequently published in the three volume Vedische Studien (Stuttgart: Kohlhammer Verlag, 1889–1901), which - unlike previous translations - avoided a purely linguistic methodology and instead took indigenous tradition into account.