Let’s be honest with ourselves: Financial infidelity is cheating. It doesn’t involve sneaking off into seedy motel rooms or sending hot and steamy text messages to a paramour, but it does involve a series of secrets and lies that can devastate a relationship. Financial infidelity is any money-related conduct that involves one spouse being less-than-truthful or forthcoming with the other spouse. It often starts off small
Source: http://www.wivestownhallconnection.com/2016/04/what-to-do-when-your-husbandwife.html

Sometimes, having a "Financial cheat" in a union might be born out of necessity, especially if one partner / spouse displays an irresponsible attitude towards money.