a ceramic
A ceramic is fired earth. Picture a clay bowl: from the first fired pot to the heat-shield tile on a spacecraft, they all belong to this word. A ceramic is an inorganic, nonmetallic solid, usually shaped from a powder and then consolidated (made solid and dense) by firing at high temperature. Its atoms are locked together by strong ionic-covalent bonds into a rigid cage, which is why a ceramic is hard, stiff, and heatproof, yet shatters instead of bending.
The word covers far more than pottery. The alumina (Al2O3) in a spark plug, the zirconia (ZrO2) in a dental crown, the silicon carbide (SiC) in a brake disc, the silica (SiO2) glass in a window, all are ceramics. What unites them is not appearance but the definition: (1) inorganic and nonmetallic in composition, and (2) usually made by firing a powder. There are honest gray areas: glass is a ceramic cooled from a melt without crystallising, and cement hardens by reacting with water rather than by firing, so a real definition always has edges.
Ceramics are the oldest engineered material (fired clay figurines are roughly 28,000 years old, pottery about 20,000) and among the most advanced (thermal-barrier coatings in jet engines, the insulator in every microchip package). This one definition, bonded this way and made this way, sets the whole chain of properties the rest of the subject unpacks: hard, stiff, refractory (high-melting), chemically inert, electrically insulating, and, above all, brittle.
Hold a coffee mug in one hand and a computer chip in the other: the glazed mug and the ceramic package around the silicon are both ceramics, inorganic solids fired from powder, even though one costs a dollar and the other guards a billion transistors.
A ceramic is defined by bonding and processing, not by looking like pottery.
A ceramic is not the same as pottery. Pottery is one small, ancient branch; the word ceramic spans clay wares, high-purity engineering oxides and non-oxides, and glasses alike.