Yup, Twitter is down. I can't recall the last time I've seen an outage like this. Although it was very frequent back in 2008 – 2010, it has been extremely stable the past couple years.
While Twitter's not 100% down, it's mostly down. It's been having fits and starts all afternoon. I guess that's like being mostly-dead, which is slightly-alive. It really depends how you're accessing Twitter (web, client, api, etc) and from where.