「以」 （not to be confused with 「似」）is a very useful character used in all sorts of words that compare time, space, or objects such as 以来、以降、以上、以下、以外、以内、以後、and 以前 . In all these words, the 「以」 essentially means “besides” and the second character indicates what to compare.
For instance, 「以外」（いがい） uses the 「外」 character for outside so it is describing anything outside of the thing we are comparing to.
田中さん以外に、行く人はいる？
– Is there person going, outside of Tanaka-san? (Is anybody going besides Tanaka-san?)
Notice how there is no particle between 「田中さん」 and 「以外」. While it is possible to insert 「の」 in between, in practice, it is more natural to directly attach the word to the end of the noun that is being compared. This applies to all the 「以」 words given above.
「以」 is an inclusive comparator
I think it’s important to mention that 「以」 means “besides [x]”, therefore, the thing that is being compared to ([x] in this case) is included in the comparison. For example, if we say 「三つ以上」, this means “three or more” and not “more than three”. Or when we say, 「明日以降」 this means “tomorrow or afterwards” not “after tomorrow”.
カードを2枚以上選んでください。
– Please select 2 or more cards.
In English, words for comparisons such as “more” and “less” implicitly exclude the thing that is being compared. People who are used to the English way of doing things need to make sure whether they need to do a little adding or subtracting before using any of the words covered here. For instance, if I wanted to say, “less than three”, I might change this to 「二つ以下」 or use some other expression such as 「未満」. Unfortunately, these comparators are not really systematic and it becomes a matter of learning vocabulary and learning when to use them based on practice.
10歳未満のお子様は乗れません。
– Children under 10 cannot go on the ride.
Here are other ways you might want to say “more than” and “less than“. Unfortunately, you can already see an inaccurate translation of 「以上」.
Sometimes, it might not be necessary to be that picky, but you should be aware of the difference for the times when it really does matter.
Finally, to prove I’m not lying, here is a similar page that explains the difference from a Japanese point of view.
Followed this link from your comment on Victory Manual. (It’s a very old post, so I thought I’d mention it.) For “less than” stuff there are similar terms in Japanese, right? 以下 and 未満, meaning “less than” (inclusive of the number stated) and “less than” (exclusive of the number stated), respectively.
According to dictionary:
数量では、基準を含んでそれより下をいい、その基準を含まないときは「未満」を使う。
There doesn’t appear to be an opposite version of 未満 for 以上 though. At least the dictionary doesn’t mention one.