From the blog post "YIMBY Party" by Alex Tabarrok (here):
The YIMBY (yes in my back yard) movement is a small but growing movement of people who are tired of ever increasing housing prices and entrenched rent-seekers who abrogate property rights and prevent development. Here is the YIMBY Party in San Francisco:
We strongly support building new housing. We have a severe housing shortage. Increasing supply will lower prices for all and expand the number of people who can live in the Bay Area.
From Yimby Town (here):