A probe. A yosu-miru move is, in some sense, a sacrifice of a stone, but is designed to yield a very sophisticated kind of information about a developing group and how best to attack it, based on its response. Yosu-miru draws on other concepts such as kikashi, aji, and korigatachi.

??(y?su) means situation or the state of things, and ??(miru) is "to see", thus "y?su o miru", to "see how things stand". In Japanese this expression is usually used to say that it's better to wait and see before taking an action (e.g. "shibaraku y?su o miru beki da", it's better to wait and see for a little while). It is not a single word or a set phrase except in Western Go literature, and "probe" is the preferred word, being self-explanatory and actually used by the speakers of its originating language.