As I did with "conservative", I take this definition from Russell Kirk's The Conservative Mind: From Burke to Eliot.
In general "a liberal" (or "a radical", as they often have been and often are still known) believes in
- "The perfectibility of man and the illimitable progress of society: meliorism. Radicals believe that education, positive legislation, and alteration of environment can produce men like gods; they deny that humanity has a natural proclivity toward violence and sin.
- Contempt for tradition. Reason, impulse, and materialistic determinism are severally preferred as guides to social welfare, trustier than the wisdom of our ancestors. Formal religion is rejected and various ideologies are presented as substitutes.
- Political leveling. Order and privilege are condemned; total democracy, as direct as practicable, is the professed radical ideal. Allied with this spirit, generally, is a dislike of old parliamentary arrangements and an eagerness for centralization and consolidation.
- Economic leveling. The ancient rights of property, especially property in land, are suspect to almost all radicals; and collectivistic reformers hack at the institution of private property root and branch.
The radical, when all is said and done, is a neoterist, in love with change."
Portrait above is "Jean-Jacques Rousseau" (1753), by Maurice-Quentin La Tour.