All the data to which this interface and its multiple underlying interfaces provide access have in common that they are relevant to heliospheric studies and/or provide solar wind input data for studies of solar wind - magnetosphere coupling, and that they are resident at GSFC/Space Physics Data Facility.
Most important among of them are Low and High res. OMNI:
(a) Low res OMNI (LRO) is Hourly "Near-Earth" solar wind magnetic field and plasma data, energetic proton fluxes (>1 to >60 MeV), and geomagnetic and solar activity indices. There are Daily, 27-day, and Yearly resolution derived from hourly data also. All details about that data descriptions and about the data access user may find at https://omniweb.gsfc.nasa.gov/ow.html.
(b) High 1-min and 5-min resolution OMNI(HRO) : Solar wind magnetic field and plasma data at Earth's Bow Shock Nose, and geomagnetic activity indices. 5-min resolution derived from 1-min data with added 5-min energetic proton fluxes. All details about high res. OMNi data descriptions and about data access user may find at https://omniweb.gsfc.nasa.gov/ow_min.html
In addition, some data sets are as reformatted or have parameters transformed to a different coordinate system to facilitate multi-point heliospheric studies (cf.COHOWeb at https://omniweb.gsfc.nasa.gov/coho/).
Most data sets are from specific instruments carried on specific spacecraft and are as provided by spacecraft instrument teams.
"About OMNI data" or equivalent options are available on many of the underlying pages. These have more detail about the data and any processing that may have been done by us.
Important NOTE about OMNI: Multiple source high and low res. OMNI data base is not “hard” product, which stay unchanged after it has been made/updated, we try to make quality improvement when the new data (mostly from Wind and ACE spacecraft) became available. Data could be swapping from one source to other as better delayed data becomes available. or one of the sources has been reprocessed by PI's. All details user may find in OMNI description at https://omniweb.gsfc.nasa.gov/html/omni_min_data.html for HRO and https://omniweb.gsfc.nasa.gov/html/ow_data.html for LRO
These doc. files include spacecraft identifiers also.
Note that a discussion of this and underlying interfaces is found at http://omniweb.gsfc.nasa.gov/html/about_interface.html

Note: OMNI has no special abbreviation, just "variety"