The podcasting industry uses the "download" to measure how many people are listening to a given show.
Every time someone plays, streams, or downloads the MP3 from the Transistor server, that counts as a download.
James Cridland, Editor of Podnews, cites:
To be fair, downloads are limited in usefulness. Some podcasts are downloaded but never listened-to; raw download numbers don’t give demographic information, either. But [they are the] global standard.
Update: Spotify streams are now included in your Transistor stats! However, Spotify has a one-day delay on how we receive stats. So your download count from Spotify will lag slightly. If you want up-to-date Spotify stats, you can use their dashboard.
On average, only 10% of podcasts get over 1,000 downloads per episode.