Speculative reason, sometimes called theoretical reason or pure reason, is theoretical (or logical, deductive) thought, as opposed to practical (active, willing) thought. The distinction between the two goes at least as far back as the ancient Greek philosophers, such as Plato and Aristotle, who distinguished between theory (theoria, or a wide, bird's eye view of a topic, or clear vision of its structure) and practice (praxis), as well as techne.
Speculative reason is contemplative, detached, and certain, whereas practical reason is engaged, involved, active, and dependent upon the specifics of the situation. Speculative reason provides the universal, necessary principles of logic, such as the principle of non-contradiction, which must apply everywhere, regardless of the specifics of the situation.
On the other hand, practical reason is the power of the mind engaged in deciding what to do. It is also referred to as moral reason, because it involves action, decision, and particulars. Though many other thinkers have erected systems based on the distinction, two important later thinkers who have done so are Aquinas (who follows Aristotle in many respects) and Immanuel Kant.