Instant vector selectors are named as such because they return a list of samples, relative to the query evaluation time, for the time series that match them. This list is called an instant vector, as it's a result at a given instant.

A sample is a data point of a time series, composed of a value and a timestamp. This timestamp, in most cases, reflects the time when the scrape occurred and that value was ingested, with the exception of metrics pushed to the Pushgateway, which, due to their nature, will never have timestamps. However, if functions are applied or operations are performed on the time series, the timestamp for the instant vector samples will reflect the query time and not the ingested time.

The way instant vectors operate – by only returning the most recent samples relative to query time that match the selector - means that Prometheus will not return time series that are considered stale (as we mentioned in Chapter 5, Running a Prometheus Server). A stale marker (a special kind of sample that marks that time series as stale) is inserted when either the originating target disappears from the discovery mechanism, or if they are not present in the scrape after the last successful one where they existed. A time series with a stale marker as its last sample will not be returned when using instant vector selectors.

Every example in the Label Matchers section was an instant vector selector, and so every result was an instant vector.