  We investigate the nature of the short-term anomaly that appears in the
lensing light curve of KMT-2023-BLG-1866. The anomaly was only partly covered
due to its short duration, less than a day, coupled with cloudy weather
conditions and restricted nighttime duration. Considering intricacy of
interpreting partially covered signals, we thoroughly explore all potential
degenerate solutions. Through this process, we identify three planetary
scenarios that equally well account for the observed anomaly. These scenarios
are characterized by the specific planetary parameters: $(s, q)_{\rm inner} =
[0.9740 \pm 0.0083, (2.46 \pm 1.07) \times 10^{-5}]$, $(s, q)_{\rm
intermediate} = [0.9779 \pm 0.0017, (1.56 \pm 0.25)\times 10^{-5}]$, and $(s,
q)_{\rm outer} = [0.9894 \pm 0.0107, (2.31 \pm 1.29)\times 10^{-5}]$, where $s$
and $q$ denote the projected separation (scaled to the Einstein radius) and
mass ratio between the planet and its host, respectively. We identify that the
ambiguity between the inner and outer solutions stems from the inner-outer
degeneracy, while the similarity between the intermediate solution and the
others is due to an accidental degeneracy caused by incomplete anomaly
coverage. Through Bayesian analysis utilizing the constraints derived from
measured lensing observables and blending flux, our estimation indicates that
the lens system comprises a very low-mass planet orbiting an early M-type star
situated approximately (6.2 -- 6.5)~kpc from Earth in terms of median posterior
values for the different solutions. The median mass of the planet host is in
the range of (0.48 -- 0.51)~$M_\odot$, and that of the planet's mass spans a
range of (2.6 -- 4.0)~$M_{\rm E}$, varying across different solutions. The
detection of KMT-2023-BLG-1866Lb signifies the extension of the lensing surveys
to very low-mass planets that have been difficult to be detected from earlier
surveys.
