"Summum bonum" is a Latin phrase which essentially translates to "the highest good." A philosophical phrase used by the ancient Greeks as well as later thinkers including theologian Joseph Fletcher, summum bonum is used in ethical philosophy to denote the supreme good to which the ethical and moral decisions and actions of all humans should be directed. While summum bonum does not particularly refer to any one thing, the idea has inspired ethical thought for generations and has been used by philosophers from Aristotle to John Stuart Mill.
In Virtue Ethics
Virtue Ethics is an approach to Normative Ethics, a branch of philosophical ethics that attempts to understand the questions that arise from the question of how society ought to act in a moral sense. Virtue Ethics concerns the virtues, or moral character of the person, rather than the rules a person should follow, his duties, or the consequences of his actions. In Virtue Ethics, summum bonum is referred to by philosopher Cicero as the highest good to which everyone and everything should refer.
Essentially, all actions and decisions, and thus everyone's moral character, are aimed at summum bonum. Summum bonum is considered something beyond the physical such as wealth or materials and beyond the spiritual such as knowledge, but an overriding good that makes the whole of human life good.
In Christian Ethics
Christian Ethics concerns itself with the moral principles set forth in the Bible and aims to use these principles to teach Christians how to act in moral and ethical situations not explicitly covered in the Bible itself. In Christian Ethics, the idea of summum bonum arises and is again refers to the supreme good, known in Christian Ethics as God. God is therefore the supreme end at which all actions are aimed, meaning the greatest good and the highest morality can only be achieved by following the will of God as written in the Bible.
In Christian Situational Ethics
A branch of Christian Ethics developed by professor Joseph Fletcher in the 1960s, Christian Situational Ethics states that other moral principles can be cast aside if the summum bonum, or ultimate good, is best served. All other laws, principles, and rules --- without exception --- are only valid if they serve the summum bonum. In Christian Situational Ethics, love, or more importantly the love of God and the concept of "love thy neighbour" referred to in the New Testament, is known as the summom bonum, rather than God himself. Called "agape," or absolute love; it is the highest good from which all general principles can thus be derived.
In Various Ethical Theories
Aristotle, Mill and Aquinas all used the idea of summum bonum in their respective ethical theories. In each of these theories, summum bonum was directly linked to happiness. Aristotle referred to summum bonum as self-realisation, which in turn will cause happiness. Aquinas thought of summum bonum simply as happiness itself, and while he does not explicitly define summum bonum as any one thing or force, Aquinas' summum bonum will always lead to joy and is always the highest good that a human can achieve. Likewise, John Stuart Mill used the idea in Utilitarianism to refer to the maximum psychological happiness possible for the maximum number of people.