In technical drawing. "Dimension line" is correct. More specifically a "dimension line indicating a feature". A single character (usually in a "bubble" or circ

In technical drawing. "Dimension line" is correct. More specifically a "dimension line indicating a feature". A single character (usually in a "bubble" or circle) would be a callout. One or more words in combination with an extension-line and an arrow-head would be a "note".