What does mm mean?
“Mm” is an informal English interjection used to mark a short pause during speech, usually while the speaker thinks, searches for words, or considers a response. Depending on intonation and context, it can instead signal agreement, acknowledgment, or that the speaker has understood. It is similar to “um,” but “mm” often feels more deliberate and can convey a thoughtful response. Written forms such as “mm-hmm” make its function clearer in conversation. In technical and measurement contexts, “mm” has a separate role as the standard abbreviation for millimetre or millimetres, the metric unit equal to one thousandth of a metre. Thus, the word’s sound-based use concerns speech and hesitation, while its written abbreviation relates to scientific and everyday measurement.
An interjection expressing a brief pause while thinking or speaking, or indicating agreement or acknowledgment.
"Mm, that makes sense."
Informal; its precise meaning depends on intonation and context.
The abbreviation for millimetre or millimetres, a metric unit equal to one thousandth of a metre.
"The wall is 20 mm thick."
Widely used in scientific, engineering, and measurement contexts.
The abbreviation remains unchanged in the plural; the expanded plural is “millimetres.”
"The two rods are each 10 mm long."
“Mm” turns a tiny pause into a complete thought, communicating hesitation, reflection, or agreement without a full sentence.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of mm
The hesitation interjection is conventionally written “mm,” but it is primarily a vocal sound and has no securely documented lexical origin. It represents a lengthened bilabial sound and functions much like “um” or “ah” in the languages and communities where it is used. In measurement writing, “mm” abbreviates “millimetre,” a word formed from Latin mille, meaning “thousand,” and metre, which comes from Greek metron, meaning “measure.”
How mm is actually used
The interjection is informal and punctuation varies with context. Its meaning may range from hesitation to thoughtful agreement, so context is important. The measurement abbreviation is standard in technical and international usage.
Easily confused with mm
Both are hesitation sounds, but “um” is chiefly a filler used while formulating words, whereas “mm” can also express considered agreement or acknowledgment.