In a quiz contest, I came across the following question: What is the term used to describe the splitting of a heavy nucleus into two lighter nuclei? Two options provided were: a) Alpha Decay b) Fission. Apparently, the correct answer is b) Fission. But even in Alpha Decay, a Helium nucleus is formed (i.e., the alpha particle), along with the daughter nucleus; both of them are obviously lighter than the parent nucleus, and so, doesn't this also fit the required criteria?
Alpha decay is treated as a special case and is usually not included in the term "fission", although it arguably is a fission (especially in the case of light elements where the alpha can represent an appreciable fraction of the original mass).
So, for the purposes of a quiz you want "fission".