A notion of nonpositive curvature for general metric spaces
  We introduce a new definition of nonpositive curvature in metric spaces and study its relationship to the existing notions of nonpositive curvature in comparison geometry. The main feature of our definition is that it applies to all metric spaces and does not rely on geodesics. Moreover, a scaled and a relaxed version of our definition are appropriate in discrete metric spaces, and are believed to be of interest in geometric data analysis. 
Miroslav Ba\v{c}\'ak, Bobo Hua, J\"urgen Jost, Martin Kell, Armin   Schikorra
math.MG