While both retinol and Retin-A contain retinoids, they differ in their concentration. Retinol and other forms, such as retinal and retinyl palmitate are found in many over-the-counter anti-aging creams. While these all contain a form of vitamin A, the concentration of retinoids is weak. Retin-A is a prescription retinoid and is a brand name for tretinoin, which is the carboxylic acid form of vitamin A and was one of the early topical acne treatments. Retin-A is sold by prescription because its acid composition is much greater than in over-the-counter products.
Undefined is a term used when a mathematical result has no meaning. More precisely, undefined values occur when an expression is evaluated for input values outside of its domain. ( 1 if no complex numbers). ( 2 if no complex numbers). ( 3 units in radians, no complex infinity). ( 4 if no complex infinity).
As in, this may have a value or meaning or definition, but we haven't gotten there yet. A lot of sites may do this with a variable X, saying it hasn't yet been defined, or we haven't found the value yet, but we can make some statements. Undefined = not defined; we have not given an explanation for what this means.