Locke uses the expressions 'mode' and 'mixed mode' as technical terms. Below are the relevant passages from Book II where he introduces the terms. See other referenced passages from Book II for more on modes and mixed modes if you're interested. (Their roles in his thought about language stem from his somewhat idiosyncratic views about knowledge and metaphysics.)
Chapter xiii: Modes
1. I have often mentioned simple ideas, the materials of all our knowledge, focusing on how they come into the mind. Now I shall discuss some of them with a different focus: this time it will be on how they relate to ideas that are more compounded, looking into the different modifications of the same idea - modifications that the mind either finds in real things or makes up on its own initiative. [A ‘modification’ of a quality is a special case of it, so squareness is a modification of rectangularity (see viii.23); and by a natural extension of that usage, the idea of squareness can be called a modification of the idea of rectangularity.]
Those modifications of a single simple idea (which I call simple modes) are as perfectly different and distinct ideas in the mind as those that are utterly unalike or even contrary to one another. For the idea of two is as distinct from that of one as blueness is from heat or as either of those is from any number; yet it is made up only of repetitions of the simple idea of a unit. Repetitions of this kind joined together make the distinct simple modes of a dozen, a gross, a million.
Chapter xxii: Mixed modes
1. In the foregoing chapters •xiii-xxi• I have discussed simple modes, showing through examples of some of the most important of them what they are and how we come by them. Now I am ready to consider the ideas that we call mixed modes. Examples are the complex ideas of obligation, drunkenness, a lie, etc., which I call mixed modes because they consist of combinations of simple ideas of different kinds, unlike the more simple modes, which consist of simple ideas all of the same kind. These mixed modes are distinguished from the complex ideas of substances by the fact that they are not looked upon to be typical marks of any real beings that have a steady existence, and are only scattered and independent ideas put together by the mind.
2. Experience shows us that the mind gets its simple ideas in a wholly passive manner, receiving them all from the existence and operations of things presented to us by sensation and reflection; we can’t make such an idea for ourselves. But mixed modes - our present topic - are quite different in their origin. The mind often exercises an active power in making these several combinations: once it has some simple ideas, it can assemble them into various complexes, thus making a variety of complex ideas, without examining whether they exist together in that way in nature. I think that is why these ideas are called notions, implying that they have their origin and their constant existence more in the thoughts of men than in the reality of things. To form such ideas it sufficed that the mind puts the parts of them together, and that they were consistent in the understanding, without considering whether they had any real being; though I don’t deny that some of them might be taken from observation. The man who first formed the idea of hypocrisy might either have taken it at first from observing someone who made a show of good qualities that he didn’t really have, or else have formed that idea in his mind without having any such pattern to fashion it by. •There must be cases of the latter sort•. For it is evident that in the beginning of languages and societies of men, some of their complex ideas . . . . must have been in men’s minds before they existed anywhere else; and that many names standing for such complex ideas were in use before the combinations they stood for ever existed.
(Translations by Jonathan Bennett)