Some folks like to characterize aikido as a circular martial art. Some folks like to talk about various styles that may be more (Aikikai, Ki society) circular or less (Tomiki) circular. Some of these folks have even characterized Tomiki aikido as being linear instead of circular (and thus, not really aikido). At first glance, it may appear that our aikido is a more flowing, circular form of the Tomiki aikido from which it evolved.
I hate to break it to y'all, but there is no such thing as circular motion in the context of human movement. People can't move in a circle. People can't even move in straight lines either, so you can't characterize aikido as linear vs. circular. People move in a series of short line segments and short arcs, zig-zagging left and right across a central tendency of direction. The closest to circular motion that we can make is standing on one foot and turning around the hip joint, which is an arc, but cannot be continued into a circle without stepping a line between arcs.
Following is a pretty good video of Bruce Frantzis 'walking a circle' in Bagua practice. I think this provides a pretty good example of what I'm talking about - movement that looks circular but if you look closer, is actually complex linear motion. Incidently, if I remember rightly, Bruce Frantzis is one of the proponents of the idea that Aikido evolved from Chinese martial arts like Bagua, or that Aikido and Bagua had common ancestors... Interesting... For more on that tangent, check out Dojo Rat's posts on Bruce Frantzis' Bagua and Aikido and Bagua.
Here's another CMA guy who is talking about 'walking in circles' and I think it is perhaps more obvious in this vid that the circles are actually lines...
Sometimes it can be a useful mental construct to imagine that your motion is circular, and not to put too fine a point on it, but if you want to be careful and precise, pay attention and you will see the same motions as lines and arcs. I find it more useful (most of the time) to work in reality (lines and arcs) than in idealized mental constructs (circles).