means a thought, notion, or mental conception — something formed in the mind, whether a plan, an opinion, or a flash of insight.

from From Greek 'idea,' meaning 'form, pattern, or appearance,' rooted in the verb 'idein,' to see — the same family that gives us 'video' and 'vision.' For Plato, an 'idea' was the perfect, eternal Form behind every imperfect thing we glimpse in the world. The word slid into Latin and then into English through the late 16th century, gradually drifting from those lofty cosmic blueprints toward the everyday notions that flicker through any ordinary head.