Meissa · λ Orionis · Orion
Third name of Mṛgaśiras, 'she who begins the year' — a fossil of the era when the year opened at the Mṛgaśiras full moon, and so a chronological datum in its own right. Amarakośa's tasminn eva ('in that very asterism') makes the equation explicit. Source of the month-name Mārgaśīrṣa/Agrahāyaṇa.
Devanāgarī from Sanskrit Wikisource; GRETIL: 'āgrahāyaṇy-aśvatthāṭ ṭhak'. Patañjali's lines read '(p_2,3.28) ka_i,455.4-456. 5 {15/44} kārtikyāḥ āgrahāyaṇī māse .' and '{37/44} kārttikyāḥ prabhṛti āgrahāyaṇī māsa iti .'
Another entry pushed back from the Amarakośa to Pāṇini. Three sūtras of Pāṇini's own: 4.2.22 āgrahāyaṇy-aśvatthāṭ ṭhak (expressly under the heading 'name of a full-moon night', 4.2.21), 4.3.50 saṃvatsara-āgrahāyaṇībhyāṃ ṭhañ ca, and 5.4.110 nadī-paurṇamāsy-āgrahāyaṇībhyaḥ. Patañjali uses it repeatedly as a live calendar term — on 2.3.28 'kārtikyāḥ āgrahāyaṇī māse' ('Āgrahāyaṇī is a month away from Kārttikī'), which fixes the interval between the two full moons and thus indirectly the position of the asterism; and on 1.1.23 he quotes the 5.4.110 phrase. Āgrahāyaṇī also appears in the gaurādi gaṇa on 4.1.41. Note that the underlying asterism, Mṛgaśīrṣa, is NEVER named in the Aṣṭādhyāyī — Pāṇini has only the derived full-moon name.