understanding who they are

Sensitivity simply means understanding who they are, their position on the issues, and how best to communicate with and influence them.

an alertness which feels everything that happens around

Sensitivity means an alertness which feels everything that happens around.

the ability of a test to recognize true positives

Sensitivity is the ability of a test to recognize true positives, while specificity measures the number of true negatives correctly identified.

true positives divided by (true positives + false negatives

Sensitivity means true positives divided by (true positives + false negatives).

a reaction that does not involve the immune system

A sensitivity is a reaction that does not involve the immune system, but still may be quite unpleasant.

that how can an instrument detect small changes in their parameters

Sensitivity is an ability of an instrument that how can an instrument detect small changes in their parameters.

the capacity to register some actually occurring phenomenon

Generally speaking, sensitivity is the capacity to register some actually occurring phenomenon.

that will change an instrument's reading

→ "Sensitivity" is the smallest amount of difference in a quantity that will change an instrument's reading.

divided by (true positives + false negatives)

Sensitivity means true positives divided by (true positives + false negatives).