Semipassive

Semipassive is an adjective used in several technical fields to describe systems that are not fully passive. Such systems have partial energy autonomy or rely on an external energy source for part of their operation, while still retaining some passive characteristics. The precise definition varies by domain, and there is no universal standard.

In radio-frequency identification, semipassive tags (often called battery-assisted passive tags) contain a small internal battery to power the microcircuit and any on-chip sensors. The tag is still activated by the reader's energy field and uses backscatter communications to answer queries, but the stored energy allows higher sensitivity, longer read range, and enhanced sensing. They are distinguished from fully passive tags, which are powered only by reflected reader energy, and from active tags, which have a larger power source and can initiate transmissions.

In astronomy, semipassive galaxies refer to systems with star formation rates below the main sequence but not yet quenched, lying in the transitional “green valley.” These galaxies typically show reduced, but nonzero, star formation and aging stellar populations. Their properties—color, spectrum, morphology—are used to study quenching mechanisms (for example, feedback from active galactic nuclei) and the timescales over which galaxies move from star-forming to quiescent states.

Because terminology is not universal, the exact meaning of semipassive can differ between fields. The common thread is partial autonomy or activity that sits between fully passive and fully active states.