- Peano discovers a space filling curve.
- St Petersburg Mathematical Society is founded.
- Heawood publishes Map colour theorems in which he points out the error in Kempe's proof of the Four Colour Theorem. He proves that five colours suffice. (See this History Topic.)
- Fedorov and Schönflies independently classify crystallographic space groups showing that there are 230 of them.
- Poincaré publishes the first of three volumes of Les Méthodes nouvelles de la mécanique céleste (New Methods in Celestial Mechanics). He aims to completely characterise all motions of mechanical systems, invoking an analogy with fluid flow. He also shows that series expansions previously used in studying the three-body problem, for example by Delaunay, were convergent, but not in general uniformly convergent. This puts in doubt the stability proofs of the solar system given by Lagrange and Laplace.
- Pearson publishes the first in a series of 18 papers, written over the next 18 years, which introduce a number of fundamental concepts to the study of statistics. These papers contain contributions to regression analysis, the correlation coefficient and includes the chi-square test of statistical significance.
- Poincaré publishes Analysis situs his first work on topology which gives an early systematic treatment of the topic. He is the originator of algebraic topology publishing six papers on the topic. He introduces fundamental groups.
- Cantor publishes the first of two major surveys on transfinite arithmetic.
- Heinrich Weber publishes his famous text Lehrbuch der Algebra (Lectures on Algebra).
- Frobenius introduces the notion of induced representations and the "Frobenius Reciprocity Theorem".
- Hadamard's work on geodesics on surfaces of negative curvature lays the foundations of symbolic dynamics.
- Hilbert publishes Grundlagen der Geometrie (Foundations of Geometry) putting geometry in a formal axiomatic setting.
- Lyapunov devises methods which provide ways of determining the stability of sets of ordinary differential equations.