In the 1920s, Carnap and Schlick both made an important use of definitions in their main publications: Schlick, in his Allgemeine Erkenntnislehre (1918, 2 nd ed. 1925) and Carnap in Der logische Aufbau der Welt (1928, mostly written by 1925). This paper first provides an analysis of the kinds of definitions which are distinguished in these books and a few other papers and then proposes a systematic comparison of Schlick's and Carnap's diverging conceptions of definitions in the 1920s, relating them, in both cases, to their respective philosophical projects in the Allgemeine Erkenntnislehre and in Der logische Aufbau der Welt.