Non-boldface

We prefer your resume on light colored paper, using standard non-italic, non-boldface type.

Source: wiktionary

We will often denote a random vector in boldface as X.[…]It is also common to use an ordinary non-boldface X and let context indicate whether X has dimension 1 or not.[…]As we progress, however, the non-boldface notation will be used with increasing frequency to match current style in the literature.

Source: wiktionary

Symbols which occur in a word denoted by a boldface letter are always denoted by the same non-boldface letter with some subscript. That is, symbols which occur in σ are denoted by σᵢ, symbols which occur in w are denoted by wᵢ, symbols which occur in X are denoted by Xᵢ, etc.

Source: wiktionary