adverb
US/ˌhɔːr.əˈzɑːn.təl.i/
UK/ˌhɒr.ɪˈzɒn.təl.i/
In a flat way, from side to side, like the line where the sky seems to meet the sea.
In context
Hold the board horizontally and slide it slowly into the gap between the two shelves.
Short entry. Full notes coming.