The writing system in which a small set of symbols — typically 20 to 30 — represents the sounds of a language rather than its syllables or words, enabling literacy at a scale and speed that no earlier writing system achieved — and reorganizing what could be thought, recorded, traded, governed, and believed.
The alphabet is so familiar that it seems natural. It is not. It was invented once — in one place, at one moment — and everything it made possible followed from that accident. Understanding the alphabet means understanding why writing itself changed what human civilization could be.