Gross Rating Point - GRP

In advertising, a gross rating point (GRP) is a measure of the size of an advertising campaign by a specific medium or schedule. It does not measure the size of the audience reached. Rather, GRPs quantify impressions as a percentage of the target population, and this percentage may thus be greater than, or in fact much greater than, 100. Target rating points express the same concept, but with regard to a more narrowly defined target audience.

GRPs are used predominantly as a measure of media with high potential exposures or impressions.
It can be calculated with:

Number of impressions/Universal number of impressions * 100

More informations on https://digiday.com/marketing/what-is-a-grp-gross-ratings-point/

Impression

An impression (in the context of online advertising) is when an ad is fetched from its source, and is countable. Whether the ad is clicked is not taken into account. Each time an ad is fetched, it is counted as one impression.

Reach

In the application of statistics to advertising and media analysis, reach refers to the total number of different people or households exposed, at least once, to a medium during a given period. Reach should not be confused with the number of people who will actually be exposed to and consume the advertising, though. It is just the number of people who are exposed to the medium and therefore have an opportunity to see or hear the ad or commercial.

Uniques

Unique people who pass through a panel in a certain amount of time.

Target rating point - TRP

Target rating point for calculation purposes is a device attached to the TV set in a few thousand viewers' houses for judging purposes. These numbers are treated as a sample from the overall TV owners in different geographical and demographic sectors. It records the time and the programme that a viewer watches on a particular day. The average is taken for a 30-day period, which gives the viewership status for the particular channel.

As in 1,000,000 impressions among the target audience / 10,000,000 people in total in the target audience x 100 = 10 TRPs). TRPs are often added up by week, and presented in a flowchart so a marketer can see the amount of impressions delivered to the target audience from each media channel.
It can be calculated with:

Number of impressions on target/universal number of impressions on target * 100

Affinity

Affinity Index is an "efficiency indicator" in media planning. It shows the weight of a specific Target Audience compared to the total population in case of a specific programme/medium.

The affinity index= TRP/GRP. In case this figure is higher than 1 (100%), it means that the programme/medium is well targeted for our Target Audience. The higher this index the better the targeting is
It can be calculated with:

TRP /GRP *100