intrinsic activity
Intrinsic activity is a simple scorecard for what a drug does once it has latched onto a receptor: how strongly it switches the receptor on, expressed as a number from 1 down to 0 and below.
By convention, a full agonist scores 1 (it produces the maximal tissue response), a pure antagonist scores 0 (binds but produces nothing), and a partial agonist falls in between, say 0.5. An inverse agonist takes a negative value because it pushes activity below baseline. This single scale lets you place any ligand on the spectrum from full activator to silent blocker to active suppressor.
Intrinsic activity is closely related to efficacy but is a relative, tissue-dependent measure: it compares a drug's maximal response to that of a reference full agonist in the same system. Because of spare receptors and differences between tissues, the same drug can look like a full agonist in one tissue and a partial agonist in another, so the number is descriptive rather than an absolute property of the molecule.
On a scale where adrenaline scores 1 at a receptor, a partial agonist producing half the maximal response would be assigned an intrinsic activity of about 0.5.
A scale from full agonist to antagonist.
Older texts use intrinsic activity (the relative maximal response, 0 to 1) almost interchangeably with efficacy, but strict efficacy is a deeper molecular property that can exceed 1.