I use the term "amateur landlord" to refer to people like myself who've bought investment properties, particularly in recent years, as a medium to long term investment, probably have not more than one or two properties, who don't rely on the income except to defray costs, and who may rely on some level of capital appreciation to determine their perceptions of asset return.
There are, by contrast, quite a number of professional landlords, who have multiple properties, the profit rent from which they rely on as a major source of income, and many (though by no means all) of whom are fully au fait with their legal obligations and tax liability.
There's also sometimes a certain pejorative quality to referring to someone as an "amateur landlord" (though since I happily refer to myself as one, you may gather I don't necessarily share the prejudice); it comes from the fact that many such recent investors remain (blissfully / wilfully ?) unaware of their legal obligations, don't register with the PRTB, don't declare income, sometimes even resent enormously any necessary maintenance and repair costs, etcetera.
I'm being rather black and white about it, but that, at any rate, is what I tend to mean when making the distinction!
) though I may be, my own tax affairs are definitely in order!