Strictly speaking a font (fount in UK) is a number of alphabets (plus a few additional characters) in one size and one type design. Just as many of the terms in our industry date from the days of hot-metal type, the word font/fount derives from the same root as foundry (French fondre) and reminds us of the pouring of molten type metal into the matrices from which the individual letter would be cast. The font would be the result of this process.

We tend to use the word font now to mean typeface — as in say Baskerville — whereas the strict definition would mean merely Baskerville Roman, Italic, Bold, Bold Italic and Small Caps in say 10 point. If the job needed 12 point too that would call for another font of the same five alphabets. There’s no fighting this shift in meaning though — the power of Apple’s drop down menu means that font means typeface now.