Reciprocity is one of the most useful (and fortunate) properties of antennas. Reciprocity states that the receive and transmit properties of an antenna are identical. Hence, antennas do not have distinct transmit and receive radiation patterns – if you know the radiation pattern in the transmit mode then you also know the pattern in the receive mode. This makes things much simpler, as you can imagine.
Reciprocity technically only applies to antennas operating in a linear medium made of linear materials (magnetic materials that exhibit hysteresis are not linear). While these types of antennas are used in practice, they are in the minority. In general, an antenna can be assumed to be a reciprocal device unless otherwise stated.