clean or pure: free of anything that stains, defiles, or disqualifies, from literal dirt to inner corruption
At its root the word means free of whatever would stain or spoil something. It can describe a thing with no dirt on it, a person the ritual law counts as fit to come before God, or a heart with nothing false or corrupt in it. One word carries all three, and the Bible moves easily from one to the next.
In the New Testament the weight falls on the inside. Jesus calls the pure in heart blessed, and warns that scrubbing the outside of a cup while the inside stays greedy misses the whole point. Paul and the other writers speak again and again of a pure heart and a clean conscience, meaning a person whose motives are undivided and whose record leaves nothing weighing on them before God.
The word also keeps the older idea of ceremonial cleanness: being in a state that lets one draw near to God rather than being shut out. The early church came to say that all foods are now clean, so the word marks a real shift in how God's people thought about purity.
A quieter use means simply innocent, or not to blame. When Paul says he is clean from the blood of everyone, he means no one's ruin can be laid at his feet.