(noun)
a claim that is made as an excuse for not doing something or a justification for why one should not be blamed for something;
e.g. Under the plea of an injury, she was able to postpone her performance.
(noun)
in legal usage, a statement made by the accused or a lawyer that responds to the charge/allegation made with a declaration of innocence or guilt;
e.g. The lawyer made a plea of innocence on behalf of her client.