= Solution
Define the <plasma parameter> as the number in a <Debye sphere>, $N_D=(4\pi/3)n_e\lambda_D^3$; some conventions omit $4\pi/3$. A good classical <plasma (physics)>[plasma] has many particles in a screening volume, $N_D\gg1$, and a system size much larger than $\lambda_D$, so collective fields dominate individual encounters.
A collision here means cumulative Coulomb deflection that significantly changes a particle's <velocity>, not necessarily a literal hard-sphere impact. A collisionless description requires the phenomenon's timescale to be short compared with the appropriate collision time, and its length scale short compared with the <mean free path>. Parametrically $\boxed{\omega_p t_c\sim N_D/\log\Lambda}$ up to convention-dependent constants, with <Coulomb logarithm> $\log\Lambda$ representing the range of <Impact parameter>[impact parameters]. Many <Oscillation>[oscillations] can then occur before collisional relaxation. The formula itself presupposes the weak-coupling regime.
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