When referring to a decision by workers to stop work, both can be used, though "walkout" may sound a bit more abrupt, and it also tends to imply that there is somewhere (typically a factory or office) to "walk out" of. For example, I'm not sure you would call a strike by taxi drivers a "walkout". "walkout" can also refer to other situations when a group of people leave a building, meeting, etc.
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