Yes, there is a specific legal definition (I did elude to that in the post you replied to).
Most people don't understand that definition and when they hear "Margin account" they assume people are making leveraged transactions which is not the case. Even Robinhood's FAQ and documentation uses the more commonly understood definition of margin. One of the primary "Features" of Robinhood gold is "Access to investing on margin". https://robinhood.com/us/en/support/articles/gold-overview/
Most people don't understand that definition and when they hear "Margin account" they assume people are making leveraged transactions which is not the case. Even Robinhood's FAQ and documentation uses the more commonly understood definition of margin. One of the primary "Features" of Robinhood gold is "Access to investing on margin". https://robinhood.com/us/en/support/articles/gold-overview/