Short answers, pulled from the story.
A chemical compound is a chemical substance composed of many identical molecules containing atoms from more than one chemical element, held together by chemical bonds. A molecule made of atoms from only one element is not a compound. Compounds contain two or more elements in a fixed stoichiometric proportion.
The four major types of chemical compound are molecular compounds held together by covalent bonds, ionic compounds held together by ionic bonds, intermetallic compounds held together by metallic bonds, and coordination complexes held together by coordinate covalent bonds. Non-stoichiometric compounds form a disputed marginal case.
Robert Boyle used the term compound in a sense similar to the modern one at least as early as 1661, when his book The Sceptical Chymist was published. He also used the terms compounded body, perfectly mixt body, and concrete. In 1724, Isaac Watts gave an early definition of chemical element in his Logick that contrasted element with compound in clear, modern terms.
A chemical formula specifies the number of atoms of each element in a compound, using standard chemical symbols with numerical subscripts. For example, water is composed of two hydrogen atoms bonded to one oxygen atom, written as H2O. Many compounds also carry a unique CAS number assigned by the Chemical Abstracts Service.
Globally, more than 350,000 chemical compounds, including mixtures of chemicals, have been registered for production and use. Each can be transformed into a different substance through a chemical reaction that breaks and reforms bonds between atoms.
A non-stoichiometric compound has reproducible proportions of its component elements that are not integral, such as palladium hydride written PdHx where x falls between 0.02 and 0.58. Many silicate minerals are called non-stoichiometric compounds, and such substances form most of the crust and mantle of the Earth.