avyayībhāva
The avyayībhāva is a compound whose first member is an avyaya (uninflected word). An example:
यथाशक्ति
yathāśakti
according to (one's) power
Defining the avyayībhāva
We first introduce another adhikāra:
अव्ययीभावः। २.१.५
avyayībhāvaḥ (2.1.5)
avyayībhāvaḥ
... is an avyayībhāva.
Then the definition begins. First, the general case:
अव्ययं विभक्तिसमीपसमृद्ध्य्व्यृद्ध्य्-अर्थाभावात्ययासम्प्रति-शब्दप्रादुर्भावपश्चाद्-यथानुपूर्व्ययौगपद्य-सादृश्यसम्पत्तिसाकल्यान्तवचनेषु। २.१.६
avyayaṃ vibhaktisamīpasamṛddhyvyṛddhy-arthābhāvātyayāsamprati-śabdaprādurbhāvapaścād-yathānupūrvyayaugapadya-sādṛśyasampattisākalyāntavacaneṣu (2.1.6)
avyayam vibhakti-samīpa-samṛddhi-vyṛddhi-arthābhāva-atyaya-asamprati-śabdaprādurbhāva-paścāt-yathā-anupūrvya-yaugapadya-sādṛśya-sampatti-sākalya-antavacaneṣu
An avyaya [used with a related subanta becomes an avyayībhāva samāsa] when the avyaya has the sense of: a vibhakti, nearness, prosperity, loss, absence, expiry, non-presence, appearance of a word, later, accordance, sequence, simultaneity, similarity, entirety, or limit.
Rule 2.1.6 is massive and allows examples like upakumbham (“near the pot”), nirmakṣikam (“without flies”), and satṛṇam (“along with the grass”). The other rules for an avyayībhāva are more straightforward:
यथासादृश्ये। २.१.७
yathāsādṛśye (2.1.7)
yathā a-sādṛśye
yathā when not in the sense of sādṛśya (similarity) [becomes an avyayībhāva samāsa when used with a related subanta].यावदवधारणे। २.१.८
yāvadavadhāraṇe (2.1.8)
yāvat avadhāraṇe
Likewise for yāvat in the sense of equal quantity;सुप्प्रतिणा मात्रार्थे। २.१.९
suppratiṇā mātrārthe (2.1.9)
sup pratiṇā mātra-arthe
a sup [used] with prati when in the sense of “a small amount”;अक्षशलाकासंख्याः परिणा। २.१.१०
akṣaśalākāsaṃkhyāḥ pariṇā (2.1.10)
akṣa-śalākā-saṃkhyāḥ pariṇā
and the words akṣa (dice), śalākā (stick), and numerals with the word pari.
Examples:
यथावृद्धम् ब्राह्मणान् आमन्त्रयस्व
yathāvṛddham brāhmaṇān āmantrayasva
Invite the brahmins who are old (vṛddha)
(2.1.7)यावत्फलम्
yāvatphalam
as many as there are fruits
(2.1.8)सूपप्रति
sūpaprati
a little bit of soup
(2.1.9)अक्षपरि
akṣapari
missed by (one) die
(2.1.10)