Mātrā | "measure"

Matra 

Mātrā (IAST)
Translation: "measure"
From Mandukya Upanishad (Verses 8, 9, 10, 11)

A Sanskrit-English Dictionary (Monier-Williams)
Sanskrit: मात्रा
Transliteration: Mātrā
Translation: "measure (of any kind), quantity, size, duration, number, degree &c."

A Concise Dictionary of Indian Philosophy
Sanskrit: मात्रा
Transliteration: Mātrā
Translation: "mode; measure; prosodial instant"
Definition:

1. The Oṃkāra is composed of the three modes, AUM, and a fourth Amātrā, silence (Turīya).
2. It is the length of time required for pronouncing a short vowel.

Mundaka and Mandukya Upanishads (Swami Sharvananda)
Sanskrit: मात्रा
Transliteration: Mātrā
Translation: "the syllable "

The Māṇḍūkya Upaniṣad (Swami Krishnananda)
Sanskrit: मात्रा
Transliteration: Mātrā
Commentary: "the syllable "


References:

1. Monier-Williams, Monier (1899). A Sanskrit-English Dictionary: Etymologically and Philologically Arranged with Special Reference to Cognate Indo-European languages. Oxford: Clarendon Press
2. Grimes, John (1996). A Concise Dictionary of Indian Philosophy: Sanskrit Terms Defined in English. Albany: State University of New York Press
3. Sharvananda, Swami (1920). Mundaka and Mandukya Upanishads: With Sanskrit Text; Paraphrase with Word-For-Word Literal Translation, English, Rendering and Comments. Mylapore, Madras: Sri Ramakrishna Math
4. Krishnananda, Swami (1996). The Māṇḍūkya Upaniṣad. Retrieved from https://www.swami-krishnananda.org/mand_0.html.