Whether Demultiplexer and Decoder are same terms or different one?
They are different terms. 74HC138 can be used both as a decoder and a demultiplexer.


As a decoder: Inputs A0, A1, A2 (many inputs) take a binary encoded signal and the result is decoded to Y0..Y7 (many outputs.) Refer to page 4(Table 3) of the datasheet you are specifying for a complete truth table.
As a demultiplexer: The input is E3 (one input) and depending on the values of A0, A1, A2 (control lines), the value of E3 is routed to one of the Y outputs (many outputs.)


The difference may appear subtle at first glance but is a very important one. In fact, decoder and demultiplexer are two of the basic concepts of digital circuits. If some websites or books treat them interchangeably it would be because the writer is careless.

This page has a quite comprehensive explanation of what a decoder is.

This page has an explanation of what a demultiplexer is.