Putting aside for the time being all the references to samatha paired with vipassana, I’d like to know exactly what the relationship between samatha and samādhi is.
Reading Ven. J’s bio of Ajahn Chah, he notes that Ajahn Chah used samatha and samādhi interchangeably (and he also used vipassana and pañña interchangeably).
CPED and PTS have:
samatha: calm; quietude of heart; settlement of legal questions. (m.)
Samatha [fr. śam, cp. BSk. śamatha] 1. calm, quietude of heart M i.33; A i.61, 95; ii.140; iii.86 sq. (ceto˚) 116 sq., 449; iv.360; v.99; D iii.54, 213, 273; DhA ii.177; S iv.362; Dhs 11, 15, 54; cessation of the Sankhāras S i.136; iii.133; A i.133; Sn 732; Vin i.5. ↔ 2. settlement of legal questions (adhikaraṇa) Vin ii.93 iv.207; cp. DhsA 144; s. paṭivijjhati Pts i.180.
– yānika who makes quietude his vehicle, devoted to quietude, a kind of Arahant; cp. Geiger, Saŋyutta trsln ii.172. – vipassanā introspection (“auto – hypnosis” Cpd. 202) for promoting calm [cp. śamatha – vipaśyanā Divy 95] S v.52; A ii.157; DhA iv.140; also separately “calm & intuition,” e. g. M i.494.
But from reading the EBT, I have the impression those are not synonyms.
from SN 46.2
(6. Samādhi)
Ko ca, bhikkhave, |
“{And} what, monks, [is the] |
āhāro an-uppannassa vā |
nutriment (for) un-arisen |
samādhi-sam-bojjh-aṅgassa uppādāya, |
concentration-awakening-factor's arising, |
uppannassa vā samādhi-sam-bojjh-aṅgassa |
(and) arisen concentration-awakening-factor's |
bhāvanāya pāripūriyā? |
development (and) fulfillment? |
Atthi, bhikkhave, |
There-is, monks, |
Samatha-nimittaṃ |
stillness-sign, |
A-by-agga-nimittaṃ. |
non-distraction-sign. |
Tattha yoniso-manasi-kāra-bahulī-kāro– |
(To) that-there, wise-mental-production-frequently-done, |
ayam-āhāro an-uppannassa vā |
is-the-nutriment (for) un-arisen |
samādhi-sam-bojjh-aṅgassa uppādāya, |
concentration-awakening-factor's arising, |
uppannassa vā samādhi-sam-bojjh-aṅgassa |
(and) arisen concentration-awakening-factor's |
bhāvanāya pāripūriyā. |
development (and) fulfillment. |
AN 5.73
in AN 5.73, if we take the command to “do jhana” as a proxy for samādhi, all 5 cases uses samatha in this way:
(5. Dhamma-dweller: has samatha, memorized dhamma but not too much V&V)
5. ♦ “idha, bhikkhu, bhikkhu dhammaṃ pariyāpuṇāti — suttaṃ, geyyaṃ, veyyākaraṇaṃ, gāthaṃ, udānaṃ, itivuttakaṃ, jātakaṃ, abbhutadhammaṃ, vedallaṃ. so tāya dhammapariyattiyā na divasaṃ atināmeti, nāpi riñcati paṭisallānaṃ, |
5. “Then there is the case where a monk studies the Dhamma: dialogues, narratives of mixed prose and verse, explanations, verses, spontaneous exclamations, quotations, birth stories, amazing events, question & answer sessions. He doesn’t spend the day in Dhamma-study. He doesn’t neglect seclusion. |
anuyuñjati ajjhattaṃ cetosamathaṃ. |
He commits himself to internal tranquility of awareness. |
evaṃ kho, bhikkhu, bhikkhu dhammavihārī hoti. |
This is called a monk who dwells in the Dhamma. |
(conclusion: do jhāna!)
♦ “iti kho, bhikkhu, desito mayā pariyattibahulo, desito paññattibahulo, desito sajjhāyabahulo, desito vitakkabahulo, desito dhammavihārī. |
“Now, monk, I have taught you the person who is keen on study, the one who is keen on description, the one who is keen on recitation, the one who is keen on thinking, and the one who dwells in the Dhamma. |
yaṃ kho, bhikkhu VAR, satthārā karaṇīyaṃ sāvakānaṃ hitesinā anukampakena anukampaṃ upādāya, kataṃ vo taṃ mayā. |
Whatever a teacher should do—seeking the welfare of his disciples, out of sympathy for them—that have I done for you. |
etāni, bhikkhu, rukkhamūlāni, |
Over there are the roots of trees; |
etāni suññāgārāni. |
over there, empty dwellings. |
jhāyatha, bhikkhu, mā pamādattha, |
Practice jhāna, monk. Don’t be heedless. |
mā pacchā vippaṭisārino ahuvattha. |
Don’t later fall into regret. |
ayaṃ vo amhākaṃ anusāsanī”ti. |
This is our message to you.” |