Employing surplus-value as capital, reconverting it into capital, is called accumulation of capital. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.410

However much the capitalist mode of appropriation may seem to fly in the face of the original laws of commodity production, it nevertheless arises, not from a violation, but, on the contrary, from the application of these laws. Let us make this clear once more by briefly reviewing the consecutive phases of motion whose culminating point is capitalist accumulation. We saw, in the first place, that the original conversion of a sum of values into capital was achieved in complete accordance with the laws of exchange. One party to the contract sells his labor-power, the other buys it. The former receives the value of his commodity, whose use value – labor – is thereby alienated to the buyer. Means of production which already belong to the latter are then transformed by him, with the aid of labor equally belonging to him, into a new product which is likewise lawfully his. The value of the product includes: first, the value of the used-up means of production. Useful labor cannot consume these means of production without transferring their value to the new product, but, to be saleable, labor-power must be capable of supplying useful labor in the branch of industry in which it is to be employed. The value of the new product further includes: the equivalent of the value of labor-power together with a surplus-value. This is so because the value of labor-power – sold for a definite length of time, say a day, a week etc – is less than the value created by its use during that time. But the worker has received payment for the exchange-value of his labor-power and by so doing has alienated its use value – this being the case in every sale and purchase. The fact that this particular commodity, labor-power, possesses the peculiar use value of supplying labor, and therefore, of creating value, cannot affect the general law of commodity production. If, therefore, the magnitude of the value advanced in wages is not merely found again in the product, but is found there augmented by a surplus-value, this is not because the seller has been defrauded, for he has really received the value of his commodity; it is due solely to the fact that this commodity has been used up by the buyer. The law of exchange requires equality only between the exchange-values of the commodities given in exchange for one another. From the very outset, it presupposes even a difference between their use-values and it has nothing whatever to do with their consumption, which only begins after the deal is closed and executed. Thus the original conversion of money into capital is achieved in the most exact accordance with the economic laws of commodity production and with the right of property derived from them. Nevertheless, the result is: 1. That the product belongs to the capitalist and not to the worker 2. That the value of this product includes, besides the value of the capital originally advanced, a surplus-value which costs the worker labor but the capitalist nothing, and which nonetheless becomes the legitimate property of the capitalist 3. That the worker has retained his labor-power and can sell it anew if he can find a buyer. Simple reproduction is only the periodical repetition of this first operation; each time money is converted afresh into capital. Thus the law is not broken; on the contrary, it is merely enabled to operate continuously. [...] Nor does it matter if simple reproduction is replaced by reproduction on an extended scale, by accumulation. In the former case the capitalist squanders the whole surplus-value in dissipation, in the latter he demonstrates his bourgeois virtue by consuming only a portion of it and converting the rest into money. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.413-414

In so far as each single transaction invariably conforms to the laws of the exchange of commodities, the capitalist buying labor-power, the laborer selling it, and we will assume at its real value; in so far as all of this is true, it is evident that the laws of appropriation or of private property, laws that are based on the production and circulation of commodities, become by their own inner and inexorable dialectic changed into their very opposite. The exchange of equivalents, the original operation with which we started, has now become turned round in such a way that there is only an apparent exchange. This is owing to the fact, first, that the capital which is exchanged for labor-power is itself but a portion of the product of other’s labor appropriated without an equivalent; and, secondly, that this capital must not only be replaced by its producer, but replaced together with an added surplus. The relation of exchange subsisting between capitalist and laborer becomes a mere semblance appertaining to the process of circulation, a mere form, foreign to the real nature of the transaction, and only mystifying it. The ever-repeated purchase and sale of labor-power is now the mere form; what really takes place is this – the capitalist again and again appropriates, without equivalent, a portion of the previously materialized labor of others, and exchanges it for a greater quantity of living labor. At first the rights of property seemed to us to be based on a man’s own labor. [...] Now, however, property turns out to be the right, on the part of the capitalist, to appropriate the unpaid labor of others or its products, and to be the impossibility, on the part of the laborer, of appropriating his own product. The separation of property from labor has become the necessary consequence of a law that apparently originated in their identity. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.412-413

The ownership of past unpaid labour is thenceforth the sole condition for the appropriation of living unpaid labour on a constantly increasing scale. The more the capitalist has accumulated, the more is he able to accumulate. Marx, Capital, Volume 1 (Progress Publishers, 1887), p.412