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An activity-period alias in radial-velocity sampling is an apparent period created when a stellar activity signal is measured at a restricted set of observing times. For observations exactly separated by $1/f_s$, a sinusoid of frequency $f$ is indistinguishable at those times from appropriate sinusoids at $|f\pm n{f}_s|$, with integer $n$. Thus a genuine stellar rotation signal can produce peaks at periods different from the rotation period.

Real observing times are irregular, so the aliases have different amplitudes and phases rather than being perfectly equivalent. Simulating a candidate signal at those same times helps test which observed peaks it predicts. The method applies to planetary signals as well as stellar noise; a periodogram peak alone does not identify a new exoplanet.^[ref]

Stellar activity is variable magnetic behaviour in a star's outer layers, including starspots, atmospheric heating and eruptive phenomena. Rotation carries regions of different brightness and line shape across the visible disc. Their changing contribution alters both the stellar light curve and the apparent Doppler shift, even when the star's centre of mass has no corresponding orbital motion.

This makes stellar activity both a subject of study and a source of stellar noise in exoplanet searches. A changing stellar spectral-line bisector tests whether a line is deforming rather than simply translating in wavelength. Simultaneous brightness measurements supply another diagnostic. Actual observing times matter because an activity-period alias in radial-velocity sampling can place an activity-related signal at a misleading period.