This strange, seemingly cryptic expression is defined in the RAE’s Diccionario del Español Jurídico como prórroga del contrato de arrendamiento de fincas rústicas o urbanas que se produce cuando, una vez terminado el contrato hecho por tiempo determinado, permanece el arrendatario disfrutando quince días de la cosa arrendada con aquiescencia del arrendador.* In that regard, reconducción refers specifically to prórroga de un arrendamiento (DLE).
Tácita reconducción has often been translated literally as “tacit renewal,” described as “renewal by default” or, perhaps in more idiomatic English, rendered as “automatic renewal (of a lease).” But it should be noted that all of these translations are actually inaccurate, confusing prórroga (“extension”) with renovación (“renewal”). Thus tácita reconducción more appropriately denotes the “automatic extension” of a lease (if, as indicated in the DEJ’s definition above, fifteen days after the contract expires neither party has given notice of termination to the other).
*Código Civil, arts. 1566-1567.