The difference in endings—-sati in Ānāpānasati and -anupassanā in Kāyānupassanā and Cittānupassanā—is due to their Pali grammatical structure and reflects a subtle distinction in meaning or emphasis, even though all three terms relate to mindfulness.

Here is a breakdown of the terms:

1. Ānāpānasati (Mindfulness of Breathing)

• Components:
  • Ānāpāna: In-breath ($āna$) and out-breath ($apāna$).
  • Sati: Mindfulness, memory, or recollection.
• Structure: It is a compound word where the whole term is essentially formed as $X$-sati, meaning "mindfulness of $X$" or, more accurately in Pali grammar, "mindfulness related to $X$" or "mindfulness by means of $X$."
• Meaning: Mindfulness of (the) in-breath and out-breath. This name directly and concisely states the object of the practice ($\bar{a}n\bar{a}p\bar{a}na$) and the faculty of mind used (sati).

2. Kāyānupassanā and Cittānupassanā

• Components:
  • Kāya: Body.
  • Citta: Mind/Consciousness.
  • Anupassanā: Contemplation, observation, or continuous looking (from anu - "after" or "along with" + passanā - "seeing").
• Structure: They are compounds formed as $X$-anupassanā, meaning "contemplation of $X$" or "observing $X$."
• Meaning:
  • Kāyānupassanā: Contemplation of the body.
  • Cittānupassanā: Contemplation of the mind.

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The Key Difference: Sati vs. Anupassanā

The core distinction lies between the terms sati and anupassanā:

Term	Meaning	Emphasis/Context
Sati	Mindfulness, recollection	The faculty or act of simple, non-reactive awareness (as in Ānāpānasati).
Anupassanā	Contemplation, observation	The process of sustained, deep investigation or insight into the nature of phenomena (as in the four Satipaṭṭhānas).

The terms Kāyānupassanā (Contemplation of the Body) and Cittānupassanā (Contemplation of the Mind), along with Vedanānupassanā (Feelings) and Dhammānupassanā (Mental Objects), are the four subjects of practice in the Satipaṭṭhāna Sutta (The Foundations of Mindfulness). The -anupassanā ending is used to denote these specific, broader categories of contemplation that constitute the full framework of establishing mindfulness.

Ānāpānasati is a single, specific technique (mindfulness of the breath) that is listed as the very first practice within the broader category of Kāyānupassanā (Contemplation of the Body). Therefore, it uses the more direct "mindfulness of..." (-sati) construction because it is the name of a singular, focused meditation practice, while -anupassanā names the entire class or foundation of contemplation.

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Similar Pali terms using the -sati ending do exist and are used, especially when referring to the faculty of mindfulness directed towards those objects, or as a specific type of practice.

The most common and directly equivalent -sati terms are:

English Translation	Anupassanā Term (Foundation/Contemplation)	-Sati Term (Mindfulness/Recollection)
Mindfulness of the Body	Kāyānupassanā (Contemplation)	Kāyagatā-sati (Mindfulness/Recollection)
Mindfulness of the Mind	Cittānupassanā (Contemplation)	Citta-sati (Mindfulness/Recollection)

Here is a brief explanation of these terms:

1. Kāyagatā-sati

• Kāya: Body
• Gatā: Gone to, related to, or directed toward.
• Sati: Mindfulness/Recollection.
• Meaning: "Mindfulness related to the body" or "Mindfulness directed at the body."

This term is found in the suttas (discourses) and often refers to a particular meditation that focuses on the body's nature, similar to the broader Kāyānupassanā. For instance, the Kāyagatā-sati Sutta (MN 119) focuses on the benefits and development of this specific mindfulness, which can include the breath, postures, and the 32 parts of the body.

2. Citta-sati

• Citta: Mind, consciousness, state of mind.
• Sati: Mindfulness/Recollection.
• Meaning: "Mindfulness of the mind."

While Cittānupassanā (Contemplation of the Mind) is the term used in the formal list of the Four Foundations of Mindfulness (Satipaṭṭhāna), the shorter form Citta-sati is grammatically correct and is used in a general sense to refer to mindfulness regarding mental states.

Summary of the Naming Distinction

The reason the canonical Four Foundations of Mindfulness use -anupassanā (contemplation/observation) is that this ending:

1. Denotes a Deeper Practice: Anupassanā implies a sustained, continuous, and analytical insight practice (Vipassanā) into the phenomena, not just a bare awareness ($\text{sati}$).
2. Defines the Foundations: The four terms (Kāya-, Vedanā-, Citta-, Dhammā-) plus -anupassanā are the primary categories, or "foundations," upon which the entire practice of Right Mindfulness is established.

The practice of Ānāpānasati (mindfulness of breathing) is so specific and foundational that it is presented as a special, distinct practice using its own clear -sati ending, and is then listed as the very first instruction under the category of Kāyānupassanā.