Have been importing in sets of 10-20 and about mid-way through the process I encountered the error “sku” not defined. Went through many iterations of checking all information and resaving .csv file. Having imported many products it ruled out the .csv file format saving process. It had to be something that changed in the new imported records.
The accented character e. Or in my case “é”.
Once removed and replaced with the standard “e” all was working fine.