What is is kissing someone else cheating?

As an AI language model, I don't have a personal opinion on what constitutes cheating. However, in general, kissing someone else while in a committed relationship is often considered a breach of trust and fidelity, leading to feelings of betrayal and hurt for the partner who learns of it. The idea of what constitutes cheating varies from person to person and from relationship to relationship, so it's ultimately up to each couple to define their boundaries and expectations around intimacy with others.