The tablespoon is a unit of volume in the US customary system and uses the symbol Tbsp.
In the kitchen, the tablespoon is the larger of the commonly used spoons served at the table and represents 1/2 US fluid ounce or, expressed in SI / metric format; 14.78676 ml.
This is often expressed as equivalent to 3 teaspoons (3 Tsp) although strictly speaking it is a little under the size of 3 metric teaspoons (5 ml) at 4.92892159375 ml. It was established to assist normal kitchens with recipes without the need for specific measuring equipment or devices. Although many spoons differ in sizes (and very rarely is a spoon filled accurately) it allowed for a decent "catch-all" and gave a "good enough" result most of the time.
The US fluid ounce is a unit of volume in the US customary unit system with the symbol fl oz.
Primarily used for measuring the volume or capacity of liquids, 1 US fluid ounce is equal to 29.57353 ml. It is also equal to 1.040843 imperial fluid ounces.
1 US fluid ounce is equal to 1/16 imperial pint, 1/32 imperial quart and 1/128 imperial gallon.
The fluid ounce was originally defined by the volume taken up by one ounce of a substance. Of course this would be different depending on the density of that substance; for example England used wine whereas Scotland used water to establish this measure.
The US fluid ounce is derived from the US gallon; which was originally based on the English wine gallon (231 cubic inches).