Rule. The words of the list beginning with svar (the svarādi), and the nipātas, receive the name avyaya, indeclinable. A nipāta is a particle as defined in chapter 1: an item of the cādi list, or of the prādi list, that does not name a substance.
The svarādi list begins: svar, antar, prātar, punar, sanutar, uccais, nīcais, śanais, ṛdhak, ṛte, yugapat, ārāt, pṛthak, śvas, divā, rātrau, sāyam, ciram, manāk, īṣat, joṣam, tūṣṇīm, bahis, avas, samayā, nikaṣā, svayam, vṛthā, naktam, nañ, hetau, iddhā, addhā, sāmi. A gaṇasūtra — a rule inside the list itself — adds vat, brāhmaṇavat, kṣatriyavat, sanā, sanat, sanāt, upadhā, tiras, antarā, antareṇa, jyok, kam, śam, sahasā, vinā, nānā, svasti, svadhā, alam, vaṣaṭ, śrauṣaṭ, vauṣaṭ, anyat, asti, upāṃśu, kṣamā, vihāyasā, doṣā, mṛṣā, mithyā, mudhā, purā, mitho, mithas, prāyas, muhus, pravāhukam, pravāhikā, āryahalam, abhīkṣṇam, sākam, sārdham, namas, hiruk, dhik, atha, am, ām, pratām, praśān, pratān, mā, māṅ. The list is an ākṛtigaṇa: an open, illustrative class, so a word of the same kind counts even if it is not printed.
The cādi list of nipātas begins: ca, vā, ha, aha, eva, evam, nūnam, śaśvat, yugapat, bhūyas, kūpat, kuvit, sūpatsa, net, cet, caṇ, kaccit, kiṃcit, yatra, naha, hanta, mākiḥ, mākim, nakiḥ, ākim, māṅ, nañ, yāvat, tāvat, tvai, dvai, rai, śrauṣaṭ, vauṣaṭ, svāhā, svadhā, tum, tathāhi, khalu, kila, atho, atha, suṣṭhu, sma, ādaha. Its gaṇasūtra upasarga-vibhakti-svara-pratirūpakāś ca adds words that merely look like (pratirūpaka) a prefix, a case-ending or a particle: avadattam, ahayuḥ, astikṣīrā; the bare sounds a, ā, i, ī, u, ū, e, ai, o, au; paśu, śukam, yathākathāca, pāṭ, pyāṭ, aṅga, hai, he, bhoḥ, aye, dya, viṣu, ekapade, yut, ātaḥ. The cādi too is an ākṛtigaṇa.
Example.
uccaiḥ → it stands in the svarādi list → it is named avyaya; nothing has to be derived.
svasti → it stands in the svarādi list (by the gaṇasūtra) → avyaya.
ca → it is a nipāta of the cādi list → avyaya.
Applies when / blocked by. A cādi word is a nipāta, and so an avyaya, only when it does not name a substance (SK 20). Bhaṭṭoji's commentators, under SK 452, add that this restriction is stricter for the cādi than for the svarādi: a svarādi word is avyaya whether or not it names a substance.
Watch out. Both lists are open. The point of printing them is to show the kind of word meant, not to close the class. Note also that the gaṇasūtra of the cādi makes the bare vowels a, ā, i and the rest nipātas when they are used as particles; do not confuse these with the vowels of the Śivasūtras.
The words svar and the rest (svarādi), and the nipātas, receive the designation avyaya (indeclinable). [The svarādi class:] svar, antar, prātar, punar, sanutar, uccais, nīcais, śanais, ṛdhak, ṛte, yugapat, ārāt, pṛthak, śvas, divā, rātrau, sāyam, ciram, manāk, īṣat, joṣam, tūṣṇīm, bahis, avas, samayā, nikaṣā, svayam, vṛthā, naktam, nañ, hetau, iddhā, addhā, sāmi; (gaṇasūtra:) vat, brāhmaṇavat, kṣatriyavat, sanā, sanat, sanāt, upadhā, tiras, antarā, antareṇa, jyok, kam, śam, sahasā, vinā, nānā, svasti, svadhā, alam, vaṣaṭ, śrauṣaṭ, vauṣaṭ, anyat, asti, upāṃśu, kṣamā, vihāyasā, doṣā, mṛṣā, mithyā, mudhā, purā, mitho, mithas, prāyas, muhus, pravāhukam, pravāhikā, āryahalam, abhīkṣṇam, sākam, sārdham, namas, hiruk, dhik, atha, am, ām, pratām, praśān, pratān, mā, māṅ. This is an ākṛtigaṇa (an open, illustrative class). [The cādi class of nipātas:] ca, vā, ha, aha, eva, evam, nūnam, śaśvat, yugapat, bhūyas, kūpat, kuvit, sūpatsa, kuvit, net, cet, caṇ, kaccit, kiṃcit, yatra, naha, hanta, mākiḥ, mākim, nakiḥ, ākim, māṅ, nañ, yāvat, tāvat, tvai, dvai, rai, śrauṣaṭ, vauṣaṭ, svāhā, svadhā, tum, tathāhi, khalu, kila, atho, atha, suṣṭhu, sma, ādaha. (gaṇasūtra:) upasargavibhaktisvarapratirūpakāśca — and [words] that resemble (pratirūpaka) an upasarga, a vibhakti-ending, or a svara (particle) [are also nipātas]. [Examples:] avadattam, ahayuḥ, astikṣīrā; a, ā, i, ī, u, ū, e, ai, o, au, paśu, śukam, yathākathāca, pāṭ, pyāṭ, aṅga, hai, he, bhoḥ, aye, dya, viṣu, ekapade, yut, ātaḥ. The cādi too is an ākṛtigaṇa.