A Relativistic speed is a speed which is a significant proportion of the speed of light. Therefore scientific analysis must take the consequences of special relativity into account. A relativistic particle is a subatomic particle moving at relativistic speed.
The boundary is not sharp. Corrections to Newton's laws go like (v/c)^2, so to get answers to 1% accuracy, a particle becomes relativistic when its speed is more than 10% of the speed of light.
See also
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