Property of a function by which they are either constantly increasing or decreasing i.e. sign of the second derivative of the function always stays the same.

University of Michigan - Ann Arbor

Some useful definitions to better understand the paper.
1. Selection Nodes
2. Functional Constraints
3. Transportability
4. Monotonicity

Helpful Definitions for the Paper

https://en.wikipedia.org/wiki/Monotonic_function

Reference : Monotonicity

1. Denoted by a solid square
2. if present, they point TO a local mechanism (say T). T is expected to differ between the environment of the experiment and the environment of application.
3. Absence of a selection node implies that that mechanism is expected to stay the same in both environments.

Selection Nodes

Used to determine if a causal relationship can be transported from one environment to another. Determined using d-separation. 
If there are a set of d measures that separate the source of the disparity from the quantity we are estimating i.e. target, then we can say that the quantity is transportable.

Transportable

A functional constraint is a property that is obtained from the nature of the problem under consideration, that must be satisfied.

Functional Constraints

https://en.wikipedia.org/wiki/Constraint_(mathematics)

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