[Tagging] Terminology primary feature, main tag, etc..

[martianfreeloader]

Hi,

I'm unsure if I'm using correct terminology. I have come across these 
terms in the OSM ecosystem:

- primary feature [1]
- main key [2]
- primary key [3]
- feature tag [4]

1) Are these synonyms (except for the key/tag distinction)?

2) Is *one* of these terms "official" OSM speek with a clear definition?
(as is the case for things like "node", "way", "relation", "key", 
"value", "tag", "changeset")


[1] https://wiki.openstreetmap.org/wiki/Map_features
[2] https://wiki.openstreetmap.org/wiki/Key:highway
[3] https://wiki.openstreetmap.org/wiki/Homonymous_keys
[4] 
https://tools.geofabrik.de/osmi/?view=tagging&lon=14.46052&lat=46.23038&zoom=15&baselayer=Geofabrik%20Standard&overlays=no_feature_tag_nodes%2Cno_feature_tag_wayshttps://tools.geofabrik.de/osmi/?view=tagging&lon=14.46052&lat=46.23038&zoom=15&baselayer=Geofabrik%20Standard&overlays=no_feature_tag_nodes%2Cno_feature_tag_ways