A "word of welcome" is usually more than one word.
It can refer to an entire (short) speech. For example, if I'm a conference chair, I might take the podium and say "Our dean will now open the conference with a word of welcome." This would probably be a short speech, no more than ten minutes, saying how happy she is to host all of the attendees on campus, how proud she is of her faculty members for organizing and hosting the conference, how she hopes all the participants will learn a lot and have a good time ...
After that, she will go back to her office and do other things.
At the other extreme, if she and her husband visit my home, my word of welcome would be something like "Come in, come in! We're so glad you could be here with us this evening!" Then we could talk about how she feels after falling off her horse, or something else.
As previously posted by George French, it doesn't have to be spoken. One can be welcomed to the WR online community just as one is welcomed to a physical place.