Understanding the Question: In the Pāṇinian system of grammar, technical names or Saṃjñās are used as shorthand to apply specific rules. We need to identify the term used for the very last sound/letter of a linguistic unit.
Key Concepts and Approach: Pāṇini defines various positions within a word. For example, Upadhā refers to the penultimate (second to last) letter, whereas the final position has its own unique designation.
Detailed Solution:
Step 1: Defining Antya. The term Antya is used in Pāṇini's grammar to signify the final phoneme (vowel or consonant) of any given word, root, or suffix.
Step 2: Application in Sūtras. Many rules in the Aṣṭādhyāyī are triggered specifically by what the antya-varṇa (final letter) is. For instance, the rule "alo'ntyasya" (1.1.52) specifies that a substitute should replace only the final letter of a term.
Step 3: Eliminating other options. Upadhā is the letter before the last. Pratyaya is a suffix. Dhātu is the verbal root itself. None of these describe a specific letter position except for Antya.
Conclusion: The technical name for the last letter is Antya.