pellucid \puh-LOO-sid\ (adjective) - 1 : Transparent; clear; not opaque. 2 : Easily understandable.
"Maybe this person is apparently mysterious while being pellucid, or apparently pellucid while being mysterious." -- Sue Miller, 'While I Was Gone'
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Pellucid comes from Latin pellucidus, "shining, transparent," from pellucere, "to shine through," from per-, "through" + lucere, "to shine."