Final answer:
A compound is a substance composed of two or more elements in a fixed ratio, bonded chemically, and can only be separated into simpler substances by chemical methods.
Step-by-step explanation:
A compound is a substance with a fixed composition of atoms of two or more different elements that are bound together. This can be seen in examples like water (H2O), where hydrogen and oxygen are present in a definite ratio. Compounds are characterized by having a uniform composition throughout and can only be separated into their individual elements by chemical methods. There is no variability in the composition of a compound; it is the same no matter what sample you take. This fixed proportion distinguishes compounds from mixtures, where the substances involved do not have specific ratio relationships and can be physically separated into their constituent parts.