rig·id 
(rĭjĭd)
(rĭjĭd)adj.
1. Not flexible or pliant; stiff: a rigid material. See Synonyms at stiff.
2. Not moving or flexing: rigid muscles.
3. Not changing or adjusting to different conditions or problems: a rigid thinker; a rigid hierarchy.
4. Scrupulously or severely maintained or performed; rigorous or harsh: rigid discipline.
5. Being an airship with a external frame made of rigid parts.
[Middle English rigide, from Latin rigidus, from rigēre, to be stiff; see reig- in the Appendix of Indo-European roots.]
rigid·ly adv.
rigid·ness n.