species – a group of similar organisms that are capable of producing fertile offspring

To remember the meaning of species, use the following mnemonic:

With great speed she's (species) jumped off the springboard (offspring) after seeing a group of similar organisms behind her. 

A species is a group of similar organisms that can breed together to produce fertile offspring, and this is the fundamental unit of classification in biology. Members of the same species share many common characteristics including similar anatomy, physiology, behaviour and genetic makeup, though there can be variation between individuals. 

The key defining feature is reproductive compatibility - organisms belong to the same species if they can successfully mate and produce offspring that are themselves capable of reproduction.

For example, all domestic dogs belong to the same species (canis familiaris) regardless of their size or appearance differences because they can all interbreed and produce fertile puppies. However, horses and donkeys are different species because although they can mate to produce mules, these offspring are sterile and cannot reproduce. 

The concept becomes more complex with organisms that reproduce asexually or extinct species known only from fossils. In these cases, scientists rely on genetic similarity, physical characteristics, and evolutionary relationships to define species boundaries.

It is crucial to understand the concept of species as it allows us to understand biodiversity, evolution and conservation.