I was flicking through some gear sites yesterday and I noticed that some of them advertise 'rucksacks', not backpacks.
Now what made this interesting was that one leading brand, Berghaus, have several different backpack options but some of them are called rucksacks while others are given the title backpack.

This got me to wondering what is the difference between a backpack and a rucksack so I did a little investigation and thought I'd add up a short post on it.
Etymology
Now, that is a big word to start a paragraph on isn't it lol 🙂 but the first place to start was to look at the source of the words. From what I can find, Rucksack is derived from Germany. 'Rucken' meaning back in German and 'Sack' (meaning bag) obviously means a sack. So, a bag for your back effectively.
If we look at Backpack, well there isn't a whole lot of difference now is there, except it being in English, it is obviously a sack (pack), bag, for your back too. The word backpack seemed to originate in the US.​
US and UK, are there differences?
I was curious as to if there were any differences and from what I can ascertain, in terms of how the words are used, not very much really. It seems that many people use the two terms interchangeably.
Broadly speaking, when looking at hiking gear providers in the US, the term backpack seems more commonly used. However, as mentioned in the comments below, it seems the US military may use the term rucksack quite a bit. I don't know anything about military equipment but it would make sense that they would need very functional backpacks / rucksacks.
I saw one mention that a rucksack is a mid sized pack used for multi day camping hikes of up to a week, and a backpack is different in terms of size and use in that it is bigger than a rucksack and is used for very long treks and trips into the wilderness, so for weeks to months. I couldn't find much to corroborate that though. On looking around, there seem to be equally large options, some referenced as a rucksack others as a backpack.​​​​​
The UK also seems to lean towards the use of the term backpack as opposed to rucksack, however, in terms of hiking gear, it did seem that rucksack could be more commonly found than in the US.
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Conclusion
So, there you have it. It seems that in terms of backpacking or hiking, there isn't really a whole lot of difference in how people use the two terms backpack and rucksack. In the US, it seems that the term backpack may be more commonly used in terms of hiking gear. However, the term rucksack is also well known.
In the UK, it also seems it is a different word for the same thing. The only mention of a difference I came across in the UK was that a rucksack is a smaller version of a large backpack used for shorter hikes and backpacking trips. However, as mentioned, I couldn't really confirm that in any viable way so I won't take it as Gospel.
All in all, in seems that in terms of usage, many people use the terms backpack and rucksack interchangeably. When it comes to hiking gear, it also seems that the word backpack is more commonly used.​​​​​
Now, I hope I have got this right and my research is correct. If not, please feel free to correct me in the comments below. I'd be interested to hear any thoughts.