Idiom

An idiom is a multi-word expression whose meaning cannot be fully predicted from the meanings of its individual words. Kick the bucket means "die," not literally kicking a bucket. Idioms are a subset of Formulaic Language: fixed or semi-fixed sequences stored and retrieved as wholes.

The Transparency Continuum

Idioms are not uniformly opaque. They range along a continuum:

Transparency	Example	Meaning	Compositionality
Transparent	spill the beans	Reveal a secret	Metaphor is accessible
Semi-transparent	break the ice	Start a conversation comfortably	Metaphor can be worked out
Semi-opaque	kick the bucket	Die	Connection to literal meaning is unclear
Opaque	by and large	Generally	No discernible literal connection

This gradient matters for teaching: transparent idioms can be taught through metaphor analysis, while opaque ones must be learned as wholes.

Properties

• Fixedness: idioms resist syntactic manipulation; She kicked the bucket but not easily *The bucket was kicked by her or *She kicked the large bucket
• Non-compositionality: meaning is not the sum of parts
• Conventionality: the meaning is agreed upon by speakers through usage, not derived from rules
• Variation: some idioms allow limited variation (let the cat out of the bag / the cat's out of the bag); others are completely frozen

Idioms vs Other Multi-word Units

Type	Example	Meaning predictable?	Fixed form?
Idiom	hit the nail on the head	No	Mostly
Collocation	make a decision	Yes (but arbitrary choice)	Flexible
Phrasal Verb	give up	Often no	Fixed verb + particle
Proverb	Don't count your chickens...	Metaphorical but a complete sentence	Fixed
Simile	as cool as a cucumber	Semi-transparent	Fixed template

Frequency and Register

• Idioms are more common in spoken English and informal writing than in academic prose
• Many idioms are register-specific: get cold feet (informal), leave no stone unturned (formal/journalistic)
• Some idioms are culture-specific and do not translate (it's raining cats and dogs has no equivalent in many languages)

Teaching Considerations

• Avoid teaching long lists of decontextualised idioms; focus on high-frequency idioms that learners will actually encounter
• Teach idioms in context, with attention to register and typical situations of use
• For transparent idioms, explore the underlying metaphor; this aids both comprehension and retention
• Productive mastery of idioms comes late; receptive knowledge (recognising idioms in reading and listening) is a more realistic goal for most learners
• Corpus data (Corpus Linguistics) can identify which idioms are genuinely frequent versus which are taught out of tradition