Velocity dependent. While spasticity has taken on many colloquial meanings, the true meaning is that of a velocity dependent resistance to stretch. A spastic muscle may be able to be stretched slowly, but when the velocity is increased, the stretch causes a reflexive contraction within the muscle to antagonize the stretch. It is very commonly seen in upper motor neuron related diseases (stroke, cp, etc).
Resistant muscle. Spasticity is complex: it is an increase in muscle resistance, typically more noticeable when a muscle is placed on stretch quickly (it is "velocity-dependent"), creating stiffness/tightness. If untreated it can lead to rigidity or contractures. It is seen most commonly in injuries/illnesses affecting the spinal cord and/or brain. Treatment includes stretching, positioning, bracing, and medication.