काष्ठीय वृक्षों के तना, शाखा, जड़ आदि के सबसे ऊपरी आवरण को छाल या वल्क (Bark) कहते हैं। 'छाल' तकनीकी शब्द नहीं है। छाल को दो भागों में बंटा हुआ मान सकते हैं - आन्तरिक वल्क और बाह्य वल्क। आन्तरिक वल्क में जीवित ऊतक होते हैं जबकि बाह्य वल्क में मृत ऊतक होते हैं।

Bark is the outermost layer of stems and roots of woody plants. Plants with bark include trees, woody vines, and shrubs. Bark refers to all the tissues outside the vascular cambium and is a nontechnical term. It overlays the wood and consists of the inner bark and the outer bark. The inner bark, which in older stems is living tissue, includes the innermost layer of the periderm. The outer bark on older stems includes the dead tissue on the surface of the stems, along with parts of the outermost periderm and all the tissues on the outer side of the periderm. The outer bark on trees which lies external to the living periderm is also called the rhytidome.