Q. 1. How does the Catholic Church define a "lax conscience?"
A. 1. On the subject of "lax conscience", the Catechism of the Catholic Church states:
# 1790 "A human being must always obey the certain judgment of his conscience. If he were deliberately to act against it, he would condemn himself. Yet it can happen that moral conscience remains in ignorance and makes erroneous judgments about acts to be performed or already committed."
# 1791 "This ignorance can often be imputed to personal responsibility. This is the case when a man "takes little trouble to find out what is true and good, or when conscience is by degrees almost blinded through the habit of committing sin." In such cases, the person is culpable for the evil he commits." ("Culpable" means being legally responsible for one's actions.)
In summary, a "lax conscience" does what is convenient versus obeying God's law, consequently the person committing venial and mortal sins by his actions. The "lax conscience" is a "careless person" who does not care about the fact that God is offended.