idiomatic expressions

noun

• 1a group of words established by usage as having a meaning not deducible from those of the individual words (e.g.over the moon, see the light).
• [mass noun] a form of expression natural to a language, person, or group of people:he had a feeling for phrase and idiom
• the dialect of a people or part of a country.

2a characteristic mode of expression in music or art:they were both working in a neo-Impressionist idiom

• a piece of cake  something easily achieved

• Costs an arm and a leg – to be very expensive

• Break a leg –means good luck!