Jacques charles described the direct relationship between temperature and volume of all gases at constant pressure. should this be called charles's law or charles's theory? explain.

This should be called as Charles law.

Law's are those which do not have any explanation or mechanism behind them, they are obtain by performing several experiments and then conclude to a one liner.

Since Jacques charles gives the relationship by performing number of experiment, there is no scientific explanation why it happens that's it should be known as Charles law.