Do "drop by" and "drop in" mean the same? are there some differences?

[answer628]

I was studying phrasal verbs, and I was curious about the difference between "drop in" and "drop by." I looked them up in the dictionary, but they seem to mean the same.

From the freedictionary.com, it says,

drop by: to visit someone casually and without being invited (I'll drop by at his house on my way home.)
drop in: to arrive informally to visit someone (Do drop in (on me) if you happen to be passing!)

If there is slightest difference, can you explain?

Thank you in advance

[5jj]

drop in, drop by and drop round all mean essentially the same.

[probus]

I can't perceive the slightest difference between drop in and drop by. But drop round is purely British, never heard in North America.