Observability is a measure of how well internal states of a system can be inferred from knowledge of its external outputs – Wikipedia
The umbrella term for how your system is doing. You'll find both "monitoring" and "instrumentation" under this term.
Monitoring is the translation of IT metrics into business meaning – Wikipedia Word to talk about tools for viewing data that has been recorded by your systems (time series data, logging etc). Monitoring helps you define the “what” and “why” something has gone wrong.
Instrumentation refers to an ability to monitor or measure the level of a product’s performance, to diagnose errors and to write trace information – Wikipedia Instrumentation is talking about how you are recording data to be viewed and monitored.
Telemetry is the process of gathering remote information that is collected by instrumentation – MSDN
Telemetry refers to the mechanisms for acquiring the data that has been gathered by instrumentation (tools like FluentD, Syslog etc). It is also referred to in some monitoring tools as breadcrumbs.