When we define a word in Sanskrit, we distinguish three levels of definition: 1. literal meaning (as found in the dictionary), 2. etymological meaning (the meaning derived from the word's root), and 3. nirukta, or "interpretive etymology," a traditional Indian exegetical reading which seeks to explain why that word is the appropriate one for what it designates. For example, for the word tantra the primary relevant meanings in the dictionary are "framework, system," as in a system of practice, "doctrine, theory," and "scripture" -- in this case, a scripture teaching a system of practice. (Note: even literal meaning is contextual, and this is why tantra never means "loom" or "weave" in a spiritual context, but only in, y'know, a weaving context.)
Number 2, etymological meaning: the verbal root of tantra is √tan, "to expand," followed by the suffix tra, which is usually an instrumental suffix. Hence tantra means "an instrument (tra) for expansion (tan)". There is only ever ONE correct etymological meaning of a word.
Number 3, nirukta: "a tantra (i.e., tantric scripture) is so called because it expands (√tan) on the topics of mantra and the principles of reality (tattvas), and because it saves (√tra) us from the cycle of suffering." That is the standard nirukta, found in the Kāmikā-tantra, but there can be more than one; and indeed, modern teachers have created their own interpretive etymologies.
Even though the basic meaning of tantra is "scripture," in time the term came to denote specific kinds of scriptures, those that we now characterize as Tantric. In this usage, the word tantra is synonymous with āgama. For a list of the specific characteristics that characterize a Tantric text, please see page 33 of the latest edition of Tantra Illuminated.