The first definition that comes back is "the cannabis plant, especially when grown for fiber."
Going from the cannabis definition we have provided, you can see how these terms start to get confusing.
Hemp is still cannabis. Hemp is simply a variety of cannabis that contains less than 0.3% delta-9 THC. This is a legal definition of a subset of the scientifically named plant 'cannabis.'
Long story short, hemp plants are low in THC and therefore not psychoactive. This is similar to the definition of CBD, because hemp contains CBD, and CBD is in cannabis, and THC is in cannabis.
All of these similar terms are more or less categorizing different chemical makeups of the same plant...cannabis.