The defenestration was that of Martinitz, Slavata, and Fabricius in Prague (then in Bohemia) on 23 May 1618. They were members of the Regency of Matthias, Holy Roman Emperor; Martinitz and Slavata were two of his three most important ministers. Matthias had left Prague in late 1617, leaving the government in the hands of the regency (which consisted of seven Catholics and four Protestants); in March 1618, an Imperial letter was issued forbidding meetings of Protestants (this was during the Counterreformation). On 23 May, Thurn, Ruppa, and Colonna de Fels met with four of the Catholic regents (in addition to Fabricius, who was there as a secretary), asking whether those four had anything to do with the edict forbidding the meetings. In the ensuing confrontation, they accused Martiniz and Slavata of alliance with the Jesuits, and proceeded to throw both of them, followed by Fabricius, from the window. Only Slavata was injured, and that was minor.

This is considered by many to be the event that sparked the Thirty Years' War.

xunker's writeup `Defenestrations of Prague' has more information on this event.