Isha Upanishad : Isha Upanishad is named so because it starts with the word Isha. The very first mantra of the Upanishad says, “Isha vasyam idam sarvam” which literally means “Isha, the Ishwara (the Almighty) exists in every nook and corner of the universe”. Isha Upanishad is in fact the 40th chapter — the last one — of the Yajurveda but in antiquity this last chapter of Yajurveda somehow came to establish its own distinct identity as ‘Isha Upanishad‘ and was identified as one of landmarks of Vedantic literature. The very tone and tenor of the 40th chapter of Yajurveda suggests that indeed it ought to have been dealt with as a separate entity. While Yajurveda (yajya+veda) deals with the processes and procedures of Vedic rituals (called karmakanda), the last chapter is purely metaphysical. Maybe that’s the reason why the sages, in antiquity, gave it a separate identity — Isha Upanishad. The 40th or the last chapter of Yajurveda contains 17 mantras while Isha Upanishad has 18 mantras, one more than Yajurveda. The order of the mantras in both of them are almost similar.
Purusharth Chatustay: The Hindu way of life comes with pre-set (or a priori) norms that a devout Hindu must follow — and these are called Purusharth Chatustay (पुरुषार्थचतुष्टय) — the four basic objectives of a human being. These are (Dharma) धर्म + (Artha) अर्थ + (Kama) कर्म + (Moksha) मोक्ष. These are the four essential objectives or goals of a devout Hindu that he must endeavour to attain.
The Ashram system: The Hindu scriptures have divided the span of human life into four phases namely; Brahmachrya ashram, Grahastha ashram, Vanaprasth Ashram and Sanyas Ashram. The system begins with the Brahmachrya ashram at the age of eight when a son enter his schooling in a ashram, a home away from home, under the guidance of a guru. Each youngster in the ashram has to decide what life he aspires to lead. The training is given accordingly. This is followed by Grahastha ashram where a person begins his life as a householder, marries, settles down as a householder, produces offsprings and helps them grow up. The third stage is Vanaprasth Ashram wherein a householders starts handing over his household responsibilities to his grown up sons, prepares to withdraw himself from material belongings and devote his time for spiritual/religious purposes. The last stage is Sanyas Ashram wherein a person leaves his household to spend time in the forests or seclusion to pursue salvation.
Karma (कर्म): The deeds done and the actions taken by an individual have the potential to bestow joy and happiness, or sorrow and setbacks, in future depending on the merit of the deeds and action taken by him/her in the present, or past. If the fruits of the deeds and actions are such that they cannot be enjoyed or suffered in the present life, an individual has to take another birth as a human being or any other living creature in order to enjoy or suffer hereafter. According to the Karma theory the mantras uttered or chanted at the sacrifices in the correct accent, in strict observance of the ritualistic details, without an error, have the magical virtue, so to say, to automatically give the desired effects or a desired object immediately, or after life, i.e. next life. Adi Shankaracharya, the 8th-century Vedic scholar in his commentary on Bhagawadagita (chapter 4, shlok 17) describes ‘karma’ as “the actions/deeds/activities/rituals performed during vedic rites like yajna or animal sacrifice. However in (chapter 4, shlok 18) he describes Karma as “any form of activity/effort” done by us to keep us alive. In fact he even treats our wilful efforts to abjure or deny action as Karma. Ramanuja (1077 CE 1157 CE) describes Karma as the actions/deeds/activities/rituals done by a person, exclusively for the attainment of Moksha.
Naishkarma Siddhi: This is a unique concept introduced in Chapter 18, shlok 49 of Bagawadagita. It literally means “negation of the consequences of Karma”.
असक्तबुद्धि: सर्वत्र जितात्मा विगतस्पृह: ।
नैष्कर्म्यसिद्धिं परमां सन्न्यासेनाधिगच्छति ॥
The above shlok literally means: He whose intellect is unattached, one who has conquered himself, and who is free from desire, attains moksha, the ultimate salvation.
The concept was picked up by Mandana Mishra, an illustrious disciple of sage Adi Shankaracharya who developed the concept in his extensive treatise named Naishkarma Siddhi (नैष्कर्म्यसिद्धिं). Mandana Mishra who followed his guru Adi Shankaracharya was designated the first Shankaracharya of the Shrengeri math and was renamed Sureshwaracharya.
Moksha: The Sanskrit word Moksha (मोक्ष), according to Sanskrit grammar, has etymologically evolved from the root verb मुच् (muc), meaning “to get rid of” or “to let loose or to free”, or “to release from”. However, the Jain tradition gives an absolutely different origin and evolution of the word. According to the Jain tradition Moksha (मोक्ष) is an amalgamation (a portmanteau) of two Sanskrit words मोह + क्षय. The closest English translation of moha (मोह) is “yearning” (for something or somebody) which leads to attachment. The other Sanskrit word Kshay or क्षय means depredation or destruction. Thus moksha (मोक्ष) would literally translate to “destruction or annihilation of yearning or attachment”. A detailed description of the word is given here.
Sat : The Sanskrit word sat (सत्) is one of the most misunderstood words among the westerners and even among the Hindus who generally equate it with another Sanskrit word, satya (सत्य), widely used to mean ‘truth’. However sat (सत्) and satya (सत्य) literally connote different meanings. The word sat (सत्), literally means “that which exists” or the “existent” or as they say in French être. As the English language word “is” is always true to its meaning in all temporal situations and all spatial positions, the Sanskrit word sat (सत्) stands true for all existent things that exist in ‘all times’ and ‘all place’, for example like the value of (pi) π or the fact that 2+2=4 is always true. So the meaning of sat (सत्) works out to mean something which is omnipresent, universal, immutable and non-temporal. Obviously all temporal and earthly things do not fall within this definition and are, therefore, not sat (सत्) or asat (असत्).
Yāska’s Nirukta (निरुक्त) and Nighantu (निघण्टु) are one of our earliest surviving systematic works on Sanskrit etymology and semantics, dated somewhere around the first millennium BCE. In simple terms Nighantu is a Vedic vocabulary/glossary while Nirukta is a systematic explanation and etymological interpretation of the words collected in the Nighantu.