The Axis powers,[nb 1] originally called the Rome–Berlin Axis, was a military coalition that fought in World War II against the Allies. The Axis powers agreed on their opposition to the Allies, but did not completely coordinate their activity.
Germany, Italy, and Japan are typically described as being the "major" (or similar) countries amongst the Axis powers. See e.g., Global strategy, Momah p.71, or Encyclopedia of World War II, Tucker & Roberts p.102.
Acceded to the Tripartite Pact, generally considered Axis powers (see e.g., Facts about the American Wars, Bowman, p.432, which includes them in a list of "Axis powers", or The Library of Congress World War II companion, Wagner, Osborne, & Reyburn p.39 which lists them as "The Axis")
Puppet states installed by the Axis Powers, see, e.g., Axis Rule in Occupied Europe, Lemkin p. 11
Official position of wartime government was that they were a co-belligerent of the Axis against the USSR and United Kingdom during the Continuation War, but generally considered to be a member of the Axis (see e.g., Bowman, p.432, Wagner, Osborne, & Reyburn p.39, or Dinardo p.95).
Declared war on the United Kingdom and United States in alliance with Japan on 25 January 1942, generally considered to be a member of the Axis (see, e.g., Bowman p. 432).
Particularly within Europe, the term "the Axis" is still often used primarily to refer to the alliance between Italy and Germany, though outside Europe it is normally understood as including Japan.
At its zenith in 1942 during World War II, the Axis presided over territories that occupied large parts of Europe, North Africa, and East Asia. In contrast to the Allies, there were no three-way summit meetings and cooperation and coordination was minimal, and on occasion the interests of the major Axis powers were at variance with each other. The war ended in 1945 with the defeat of the Axis powers and the dissolution of their alliance. As in the case of the Allies, membership of the Axis was fluid, with some nations switching sides or changing their degree of military involvement over the course of the war.