Final answer:
Personal or informal knowledge is referred to as tacit knowledge, which is directly acquired through personal experience and is not easily expressed through words.
Step-by-step explanation:
Personal or informal knowledge is referred to as tacit knowledge. Tacit knowledge represents knowledge acquired through personal experience and direct acquaintance, which is not easily transferable through writing or verbalization. This includes skillsets and insights people develop over time, like knowing how to ride a bicycle or recognizing the face of a friend. British philosopher Bertrand Russell made a distinction between tacit knowledge, termed knowledge by acquaintance, and propositional knowledge, otherwise known as knowledge by description. While propositional knowledge can be expressed through factual statements, tacit knowledge is more intuitive, empirical, and based on the individual's personal experiences.