Unobserved/unmeasured are equivalent in meaning but we might prefer tp say one over the other based on study type. Unmeasured pertains to when you could have taken a measurement (usually experiments or surveys), unobserved when you don’t have that in the data (usually secondary data). Either results in a column you wish you had, so they’re the same.
Omitted is when you turn to running a model. It is usually omitted because you don’t have the column, but you can omit a variable you do have in the data and then rectify that.
Missingness is completely different and is about cells, not columns, lacking data. “We are missing that data” is in line with the post but is not the same as the problem of “We have ‘missing data.’”
Related: The Mere Deadline Effect