別に～ない means 'not really' or 'not particularly.' It is often used to respond to a question or to soften a statement, implying that something is not the case to a significant degree. For example, if someone asks if you like coffee, you might say 別に好きじゃない (I don't particularly like it), meaning you don't have a strong positive or negative feeling.

This pattern is common in casual conversation. You can use it with verbs, adjectives, and nouns, but the negative form is essential. The nuance is that the speaker is downplaying something, often sounding a bit indifferent or nonchalant. It's different from just saying 'not' because it adds a sense of 'not especially' or 'nothing really.'

Be careful not to overuse it in formal writing, as it sounds conversational. In polite speech, you can still use it, but the negative form should be polite (e.g., 別に好きじゃないです).