In my book it specifically says that for decoders only one output is high for combinations of each input. And also the number of outputs are 2 power of number of inputs. None of these are true for BCD to 7 segments decoder. It has 4 inputs and 7 outputs and has more that one high for the outputs of each input in its Truth Table. So is calling it a decoder wrong?
It can decode BCD and drive segments of a 7 segment display.
So both depending on how you think of the device. It clearly has to decode BCD or binary into one out of 10 or 16 states and then encode that into 7 outputs for the display.
There are also chips that decode only. Such as binary 3-to-8 decoder like the 74138 or maybe even BCD 4-to-10 decoder exists.
Your book is making a statement about binary to 1 of N decoders.
It is true within the context it is intended to cover.
The statement does not apply to other types of decoders.
And also the number of outputs are 2 power of number of inputs.
This means that if you have e.g. 2 inputs then there are 22 possible combinations of input (00 01 10 11) so that there are 4 possible outputs.
Again, this is true only in the context intended - which is binary decoders.
A better statement would have been " ... the maximum number of outputs are a power of 2 of the number of inputs."
As an example where this is not true, a CD4017 has 4 inputs so a maximum of 16 outputs - but actually has only 10 outputs. This is useful in many cases and allows the use of a package with fewer pins than a 4 input to 1 of 16 decoder.