What is the term for two or more atoms that are joined tightly together?

The term for two or more atoms that are joined tightly together is "molecule." A molecule can consist of two or more atoms of the same element (as in the case of oxygen, which is O2), or it can consist of atoms from different elements (as in water, H2O). The defining characteristic of a molecule is that the atoms are bonded together by chemical forces, which gives the molecule its distinct properties.

In contrast, an element refers to a pure substance that cannot be broken down into simpler substances. A compound is a type of molecule that specifically consists of two or more different types of atoms chemically bonded together. A mixture, on the other hand, involves combinations of substances that are not chemically bonded, allowing them to retain their individual properties. Thus, while "compound" could also be a relevant concept, particularly if different atoms are involved, the most direct definition for a tightly joined entity of atoms is "molecule."