Etymology and nomenclature
The word Durga (दुर्गा) literally means "impassable", "invincible, unassailable". It is related to the word Durg (दुर्ग) which means "fortress, something difficult to defeat or pass". According to Monier Monier-Williams, Durga is derived from the roots dur (difficult) and gam (pass, go through). According to Alain Daniélou, Durga means "beyond defeat".
The word Durga and related terms appear in the Vedic literature, such as in the Rigveda hymns 4.28, 5.34, 8.27, 8.47, 8.93 and 10.127, and in sections 10.1 and 12.4 of the Atharvaveda.[note 1] A deity named Durgi appears in section 10.1.7 of the Taittiriya Aranyaka. While the Vedic literature uses the word Durga, the description therein lacks the legendary details about her that is found in later Hindu literature.
The word is also found in ancient post-Vedic Sanskrit texts such as in section 2.451 of the Mahabharata and section 4.27.16 of the Ramayana. These usages are in different contexts. For example, Durg is the name of an Asura who had become invincible to gods, and Durga is the goddess who intervenes and slays him. Durga and its derivatives are found in sections 4.1.99 and 6.3.63 of the Ashtadhyayi by Pāṇini, the ancient Sanskrit grammarian, and in the commentary of Nirukta by Yaska. Durga as a demon-slaying goddess was likely well established by the time the classic Hindu text called Devi Mahatmya was composed, which scholars variously estimate to between 400 and 600 CE. The Devi Mahatmya and other mythologies describe the nature of demonic forces symbolised by Mahishasura as shape-shifting and adapting in nature, form and strategy to create difficulties and achieve their evil ends, while Durga calmly understands and counters the evil in order to achieve her solemn goals.[note 2]
There are many epithets for Durga in Shaktism and her nine appellations are (Navadurga): Shailaputri, Brahmacharini, Chandraghanta, Kushmanda, Skandamata, Katyayini, Kaalratri, Mahagauri and Siddhidatri. A list of 108 names of the goddess are recited in order to worship her and is popularly known as the "Ashtottarshat Namavali of Goddess Durga".
Other meanings may include: "the one who cannot be accessed easily", "the undefeatable goddess".
One famous shloka states the definition and origin of the term 'Durga': "Durge durgati nashini", meaning Durga is the one who destroys all distress.