score:3
Shamatha and Samadhi are not synonyms.
Samadhi is Single-Pointed Concentration, a completely stable and highly refined state of attention. You can achieve voluntary sustained Samadhi through meditation practice, however one could also accurately call Samadhi to, for example, as when you are heavily in lust or infatuated with an object, where your attention is single-pointedly concentrated on the object of your desire, or when you are immersed in playing a video game. Samadhi just refers to a type of attention which is single-pointed, stable, and non-distracted, and is not good or bad by itself.
Shamatha, on the other hand, means "mental quiescence" or "calm abiding", and it refers to a state of the mind which is fully pliable and yelding. It is composed of two main components: the conscious and voluntary direction of attention with a powerful and enhanced awareness (sati a.k.a. mindfulness), and stable, single-pointed attention (samadhi).
Therefore, Samadhi is an essential component of Shamatha but they are not the same thing.
Upvote:2
Samatha is in group of two, Samatha & Vipassanā. It is by function.
Samādhi is in group of three, Sila, Samādhi, Paññā. It is by state (step by step of practices [sikkhā]).
Every wholesome which one does is Samatha if one doesn't do Vipassanā because it is just periodical calming by calming function, although it is completed.
Vipassanā is Seeing truth function. The truth covers every reality, so it is forever functions after it is completed.
Dāna&Sīla are not stable.
Samādhi is stable but not understand enough to complete seeing reality function.
Paññā is seeing (Ñā) every angles (Pa) of realities. There are many realities and each reality has many angles, so if the practitioner's Samatha (calming function) is not Samādhi (stable) enough, it will has not enough time to collect enough angles to see the truth. It is going to lost strong power of wholesome enough to see the truth, and need more time to begin for collecting the mind power again.
These all are cleared in NettiPakarana.
Upvote:4
If I sit dow on the cushion for a moment, that's momentary samatha. Samatha is simply calmimg. It can take place anywhere. Samatha can lead to samadhi, but the two are by no means identical. It would be akin to equating the ingredients to a fully baked cake.