Ok. Thank you hawhill and knc1.
Now I use epub file format, I have realized that selecting words to get a dictionary definition does not work as good as in fb2 files.
I mean, in the same book in fb2 and epub file formats, the word "Osiris" in the same paragraph is full marked in fb2 but only "iris" in epub file. In other paragraphs the same word, "Osiris", is full marked in both files.
I have seen this problem in other words in this book, not only in "Osiris".
If you consider this as a bug I will report it in
https://github.com/koreader/koreader/issues