[Steven Lyle Jordan]

Going for three:

Science Fiction is subdivided into layers, defined generally as "hard" and "soft," or "SF" (hard) and "Sci-Fi" (soft). The level of "hardness" or "softness" depends on the likelihood of the science/physics in the story to be considered (or proven to be) accurate, with "hard" applying to higher likelihood, and "soft" applying to lower likelihood.

The use of the word "likelihood" is key here, as SF is essentially a speculative genre, and often deals with concepts that cannot be proven, or may not be proven for decades or more. Without using the word "likelihood," basically any unproven SF would automatically be "soft" or "Sci-Fi."

Generally speaking, when SF elements of various likelihoods are present, a story's "hardness" is usually rated down to the SF element that has the lowest likelihood of accuracy. These labels, as indicated by others, depend solely on how much detail (if any) is given to the science/physics of the story, and in some cases, may not apply to an SF story at all.