specialized cell in the angiosperm phloem derived from the same parent cell as the closely associated. immediately adjacent sieve-tube member.
Merriam-Webster Online Dictionary
companion cell (noun) a living nucleated cell that is closely associated in origin, position, and probably function with a cell making up part of a sieve tube of a vascular plant
companion cell (Wikipedia)
Cross-section of a flax plant stem:
Phloem (pronunciation: /ˈfləʊ.əm/) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as photosynthates, in particular the sugar sucrose, to parts of the plant where needed. This transport process is called translocation. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word φλοιός (phloios) meaning "bark". The term was introduced by Nägeli in 1858.