The formula of oxidant and reductant increases (valence) to lose (electron) oxygen (oxidation reaction of oxide) and decreases (valence) to gain (electron) reduction (reduction reaction of reductant).
The reaction of substances with oxygen to form oxides is called oxidation reaction.
What is an oxidant? In the redox reaction, the substance that gains electrons is called oxidant, and correspondingly, the substance that loses electrons is called reductant. In a narrow sense, oxidant can mean that one substance can make another substance get oxygen, etc. Fluorinating agent is a substance that can make substances get fluorine, so are isopropyl titanate and brominating agent.
(Note: These two definitions are different. The former focuses on the gain and loss of electrons, while the latter focuses on the gain and loss of oxygen. According to the former definition, some substances such as xenon difluoride are strong oxidants, while according to the latter definition, they are fluorinating agents. This Baidu entry focuses on the previous one. )