Element
An element is a pure substance made of only one kind of atom; it cannot be broken down into simpler substances by chemical means.
Definition
An element is a substance made of only one kind of atom, and you cannot break it into anything simpler by heating, mixing, or burning it. More precisely, an element is a pure substance whose atoms all share the same atomic number $Z$; each has a one- or two-letter symbol and a fixed position on the periodic table. Elements can exist as single atoms (helium, $He$), as molecules of one kind of atom (oxygen, $O_2$), or as extended solids (iron, $Fe$), and a sample may include several isotopes and several allotropic forms with different bonding arrangements. Of the $118$ known elements, about $94$ occur naturally and the rest have been synthesized, and their properties vary periodically with $Z$ because valence electron configurations recur.
Example
Gold, oxygen, iron, and carbon are elements, and pencil "lead" is really graphite, which is pure carbon. Oxygen gas is $O_2$, two oxygen atoms bonded together, but still an element because only one kind of atom is present, whereas water, $H_2O$, contains both hydrogen and oxygen and is a compound. Carbon shows how one element can take very different forms: diamond (each atom bonded tetrahedrally to four others, extremely hard), graphite (flat sheets of hexagons, soft and conductive), and fullerenes such as $C_{60}$ are all the element carbon.
Key Insight
There are only $118$ known elements, but they combine into millions of substances, the way $26$ letters make every word. Some symbols come from Latin names, $Fe$ for iron (ferrum), $Na$ for sodium (natrium), $Pb$ for lead (plumbum), and $Au$ for gold (aurum), so the symbol is international even when the name is not. Elements heavier than iron are not made by normal fusion in stars, since fusing them absorbs energy instead of releasing it; gold, uranium, and most of the heavy elements in your body were forged in supernovae and neutron star mergers.