Optimism is the tendency to expect things to turn out well, while idealism is holding onto high, sometimes unrealistic, principles about how the world should be. An optimist expects the glass to fill up; an idealist has a picture of exactly what should be in it.
The core difference
Optimism is about expecting good outcomes; idealism is about holding high standards or principles for how things ought to be.
- optimism — a tendency to expect good things to happen: Despite the setbacks, she never lost her optimism.
- idealism — holding high, sometimes unrealistic, principles or hopes about how things should be: Her idealism made her want to fix every problem in the world at once.
How to tell them apart
Optimism is forward-looking and outcome-focused: it is a stance on what will happen. Idealism is principle-focused: it is a stance on what should happen, regardless of whether it is likely. Someone can be an idealist without being especially optimistic that their ideals will ever be reached.
Optimism comes from Latin optimus, "best," originally tied to the philosophical claim that this is the best of all possible worlds. Idealism, first recorded in English around 1796, is simply "ideal" plus "-ism." In philosophy, idealism also has a separate, technical meaning: the view that reality is shaped by the mind.
