I've seen it defined, perhaps ironically, as "When the government takes over the corporations, that's called communism. When the corporations take over the government, that's called fascism."

I always thought of it as basically a reaction to communism [...]

From Jack's link in the previous comment:

By the time Mussolini returned from Allied service in World War I, he had decided that socialism as a doctrine had largely been a failure. In 1917, Mussolini got his start in politics with the help of a £100 weekly wage from MI5, the British Security Service; this help was authorised by Sir Samuel Hoare. In early 1918, Mussolini called for the emergence of a man "ruthless and energetic enough to make a clean sweep" to revive the Italian nation. Much later in life Mussolini said he felt by 1919 "Socialism as a doctrine was already dead; it continued to exist only as a grudge". On 23 March 1919, Mussolini reformed the Milan fascio as the Fasci Italiani di Combattimento (Italian Combat Squad), consisting of 200 members.

No further comment. :)