These words all represent very similar meanings (mainly describing how something is "shared" or "typical" among others of it's kind, or within a predefined group) and many of them can be used interchangeably. If you would like to know specifics on each one, though, I'll try my best. 1. Common: In this case, common describes something that is shared or typical among a group. For example, "It is common for people to have breakfast in the morning before going to work or school," or "The two countries share a common interest in energy development." 2. Usual: This word is more associated with something that is either customary or known to be "common" because it has been observed as such over some period of time. For example, if someone frequently visits the same restaurant, they might ask the waiter to have "the usual," meaning that the waiter already knows what meal they want because of past experience. Another example, in a different context, would be, "The usual weather this time of year is bright and sunny." 3. Average: This is similar to "common," except it is more frequently used with mathematical statistics, numerical values, or ratings. For example, "The average rainfall this year was 60 cm." 4. Ordinary: This describes something that is regular, normal, or not of interest because of it's lack of anything special to separate it from a crowd. For example, "Among all of the ordinary people in the crowd, I could spot my friend easily because of her brightly colored clothing and purple hair. She is definitely not ordinary when it comes to appearances." 5. Plain: Very similar to ordinary in that it can mean "normal," however, it can also describe something clearly presented. Dictionary.com has a good description: "free from ambiguity or evasion; candid; outspoken: the plain truth of the matter." I think when compared to "common, usual, average," the meaning you are looking for is closer to describing something that isn’t noteworthy, or is just normal. For example, "He wore a plain suit and tie to work every day and worked quietly at his desk. He never did anything to stand out from the other employees." I hope this helps :) I don't think I can cover every possibility of the differences between these words without making this extremely long, so feel free to contact me if you need more help.