The word chancel derives from the Latin cancelli, meaning lattice screens or grilles, a name that points directly to the physical barrier, a rood screen, altar rail or similar lattice-like structure, that historically separated this clergy space from the nave, making the chancel's very name a description of the boundary rather than of the space it encloses.
Medieval Western churches typically maintained a fairly firm division of this kind, often with a solid rood screen carrying a large crucifix and blocking most direct sightlines from nave to altar, while the degree of separation eased considerably in many traditions after the Protestant Reformation and again after twentieth century Catholic liturgical reform, both of which, for different theological reasons, favored greater visibility and participation of the congregation in what took place at the altar.
Eastern Orthodox churches took a different path from the same starting point, replacing any partial screen with the iconostasis, a solid wall of icons that fully separates the sanctuary from the nave while communicating through image rather than direct sightline, so that the chancel's degree of physical openness today varies enormously across Christian traditions even though nearly all of them retain some version of a distinct, named space reserved around the altar.