  Combined with very-long-baseline interferometry measurements, the
observations of fast TeV gamma-ray flares probe the structure and emission
mechanism of blazar jets. However, only a handful of such flares have been
detected to date, and only within the last few years have these flares been
observed from lower-frequency-peaked BL~Lac objects and flat-spectrum radio
quasars. We report on a fast TeV gamma-ray flare from the blazar BL~Lacertae
observed by VERITAS, with a rise time of $\sim$2.3~hr and a decay time of
$\sim$36~min. The peak flux above 200 GeV is $(4.2 \pm 0.6) \times 10^{-6}
\;\text{photon} \;\text{m}^{-2}\; \text{s}^{-1}$ measured with a
4-minute-binned light curve, corresponding to $\sim$180\% of the flux which is
observed from the Crab Nebula above the same energy threshold. Variability
contemporaneous with the TeV gamma-ray flare was observed in GeV gamma-ray,
X-ray, and optical flux, as well as in optical and radio polarization.
Additionally, a possible moving emission feature with superluminal apparent
velocity was identified in VLBA observations at 43 GHz, potentially passing the
radio core of the jet around the time of the gamma-ray flare. We discuss the
constraints on the size, Lorentz factor, and location of the emitting region of
the flare, and the interpretations with several theoretical models which invoke
relativistic plasma passing stationary shocks.
