Monitoring
Is a systematic process of collecting and recording the activities taking place in target system, application or service and then using that recorded values to check if the targets are reaching their objectives or not,
Alert
An alert is the outcome of an alerting rule in Prometheus that is actively firing. Alerts are sent from Prometheus to the Alertmanager.
Alertmanager
The Alertmanager takes in alerts, aggregates them into groups, de-duplicates, applies silences, throttles, and then sends out notifications to email, Pagerduty, Slack etc.
Target
A target is the definition of an object to scrape. Target is an object whose metrics are to be monitored.
Instance
In Prometheus terms, an endpoint you scrape is called an instance. Example: 1.2.3.4:5670, domain.tld:9999 etc.
Job
A collection of targets with the same purpose
Sample
A sample is a single value at a point in time in a time series.
In Prometheus, each sample consists of a float64 value and a millisecond-precision timestamp.
Example: http_requests_total{method="get"} which have a value 32