In Klaus Mylius' Dictionary of Old-Indian Ritual, tantra is defined as follows: "Grundform, Regelwerk einer Opferkategorie; so ist das Neu- und Vollmondopfer zugleich tantra für alle iṣṭis". In Mīmāṃsā, on the other hand, it indicates the performance of direct subsidiary rites just once (although they apply to all parts of the main sacrifice). Pārthasārathi Miśra defines it as follows: "tantra is indeed the common performance of the subsidiaries, like the pre- and post-sacrifices in regard to the offerings of the [rice-cake] to Agni etc. (tantraṃ nāma sādhāraṇam aṅgānuṣṭhānam –yathāgneyādiṣu prayājādīnām, AN V, xi adhyāya, intro ad 1, p. 295). That is, the pre-sacrifices are performed just once, but they apply "in common" to all offerings.
I am grateful to Dr. Tiziana Pontillo who pointed out a definition of tantra in Lāṭyayana Śrauta Sūtra: bhūyiṣṭhaṃ tantralakṣaṇam (VI.9.13). This can still be consistent with the idea of tantra as prakṛti ("the characteristic of tantra is to be found in many rituals"). However, from this one can also develop a different concept of tantra. In fact, the sūtra could also be interpreted as "the characteristic of tantra is that [what is performed by means of it] is present (that is, valid) in many [offerings]".