From 2007.igem.org

Organism Organisms are living complex adaptive system composed of cells: complex because they are diverse and made up of multiple interconnected elements and adaptive because they have the capacity to change and learn from experience (Interdisciplinary Santa Fe Institute).

• Multicellular/unicellular

Some organisms, such as bacteria or protozoa, are unicellular. Other organisms, such as humans, are multicellular.
We will define a multicellular organism as the necessary and sufficient existence of at least 2 cell types. It can be discussed whether the grouping of identical cells constitute a multicellular organism. For example, E. coli is a bacterium able to form colonies but is a unicellular organism as a single cell is able to survive on its own if separated from the colony. On the contrary, the simplest living multicellular organisms, *sponges, consist of four specialized cellular types unable to survive alone if separated: Choanocytes, the digestive cells; Sclerocytes, support-structure-secreting cells; Porocytes, tubular pore cells; and Pinacocytes, epidermal cells. Colonial organisms, such as Volvox, considered being an evolutionary intermediate between the unicellularity and multicellularity. They are commonly classified as unicellular because individual cells are free-living and can survive on their own if separated from the colony. Since there is no consensus on the subject, our design of a synthetic multicellular organism will include two cells types and thus compel with the above definition.
Another basic property of multicellular organisms is the occurrence of cellular differentiation leading to complementary specialized functions and interdependency between different cell types.

• Cellular differentiation / cell type

Cellular differentiation is the process by which a cell acquires a new cell type. A cell type is a distinct functional and/or morphological form of a cell. A cell that is able to differentiate into many cell types is known as pluripotent. Generally, differentiation leads to progressive restriction of the developmental potential and increased specialization of function. (NCBI MeSH).
Differentiation needs signalling between and within cells leading ultimately to changes in gene expression patterns: some genes are turned on and other are turned off. Considering how gene expression may change, we can distinguish 2 forms of differentiations. Most of the time, it is epigenetic differentiation where mechanisms do not involve changes to DNA sequence: "classical" cis and trans gene regulation involving transcription factors, enhancers, repressors but also DNA methylations, histone modifications, DNA replication timing... The second form of differentiation involves changes to DNA sequence by recombination processes, for example during lymphocyte differentiation (V-D-J recombination).

Germ line / Somatic line

In multicellular organisms, we can observe two categories of cells. Germ cells are responsible for the reproduction of the organism and the others, somatic cells, are unable to build a new organism. The germ line is also totipotent (able to built an entire organism) and pluripotent (gives all cell types). But why the unit of reproduction (propagule) has to be as small as possible (a unique cell = an egg)? The proposed advantages are multiple.
First it enables to have genetic uniformity in whole organism and also reduces conflicts between cells. Second it enables to evolve significant changes in the evolution of the organism. Indeed if several cells are needed to reproduce, there is little chance to have the same mutations in every cell type, so individual cells will have different genes and significant changes will be difficult. It is also only through a coherent developmental program with the same genes that organisms can evolve: an easy way is that every cell comes from an egg.
We can notice that there are multicellular organisms, such as the cellular slime mould that develop by aggregation and not from an egg, but their pattern of behaviour have remained very simple for hundred of millions of cells. The evolution of more complex organisms increases the pressure to use an egg as propagule. In an evolutional point of view, the organization of the organism is also focused on the germ cells (the propagules). If the organism has several specific and important needs, it enables each somatic cell type to evolve to a single specific and efficient function that would require too much energy for one cell type: cellular differentiation.
(Source: Wopert, 2002).