Actually, it should be "E Ink," because "E Ink" is the trade name of E Ink Corporation, which created the technology. You can't have a space in a tag name on Stack Exchange sites, however, so e-ink would be the correct tag, not eink.
I agree for exactly the reason Ed Cottrell has said. I have created a tag synonym that will map the eink tag to e-ink. I have also merged any existing questions (there were 11 at the time) so that the new tag e-ink is displayed instead of the old tag.