The propensity interpretation of probability defines probability as the "propensity", or physical dispostion, inherent in the object or situation. For example, the propensity of a die to show a six.
Probability = longrun relative frequency for that object or situation Problems:

'[P]robability is a more or less primitive idea that measures the physical propensity or dispostion of a given kind of physical situation to yield an outcome of a given type, or to yield a "longrun" relative frequency of an outcome of a given type.'
Audi (1999)
propensity, an irregular or nonnecessitating causal disposition of an object or system to produce some result or effect.
Audi (1999)
'The propensity theory, or at least one of its versions, takes probability to be a propensity inherent in a set of repeatable conditions. To say that the probability of a particular outcome is p is to claim that the repeatable conditions have a propensity such that, if they were to be repeated a large number of times, they would produce a frequency of the outcomes close to p.'
Gillies (2000)
"The propensity theory, substituted by Popper for the frequency theory, defines probability as a propensity of objects themselves, e.g. of a die to show a six. Popper claims propensities are no more ‘mysterious’ than gravitational fields, but one can still ask just what propensities are and how wide an area the theory covers. The word ‘chance’ can also be used for ‘propensity’, and for objective probability when this is distinguished from subjective degrees of belief [...]."
Lacey (1996)