I think "traction" and "tail" are fundamentally different concepts. I see people using them interchangeably, but I believe that understanding the difference and using the terms consistently is important when trying to sell books on a variety of retailers (i.e., "wide"). Understanding the difference is important because not all retailer sites are equal and it's important to understand what can be expected on any given site.
So, what do these terms mean? The following is how I would define them. Feel free to leave a comment to disagree, suggest refinements, etc.
"Tail" can refer to a couple things. It can mean the long-term residual effect from a marketing tactic, and it can refer to long tail distributions. See "Long tail" on Wikipedia for an in-depth discussion of the latter. Although long tail distributions definitely matter to indie artists of all kinds, in indie book marketing most people will use "tail" to mean the former, i.e., the ongoing positive impact from a specific marketing tactic—maybe a BookBub feature, maybe a blog tour (does anyone do those anymore?), or whatever.
Traction refers to making lasting progress. Think of hiking up a steep or snowy slope. If you eventually slide back down, you didn't have traction. That's why I say that a tail is not traction. A marketing tactic (BookBub, etc.) might be the start of getting traction, but it might not, even if it has a good tail.
Read on to see why this matters when selling books "wide."