Definition of Phenotype
Genotype:

Simply genotype is the inherited genetic information an individual has, and is conceptually quite easy to grasp. For example, if height is determined by gene $A$ and there are two alleles (variants of the gene, $A_1$ and $A_2$), then the three possible genotypes at that locus are $A_1A_1$, $A_1A_2$, and $A_2A_2$.

Phenotype:

The definition of a phenotype is a little more difficult to grasp, in part because it's so broad. As a quantitative geneticist I would say its any characteristic of an individual that can be measured. There's an enormous variance in the phenotypes that are used, they can be characteristics that are physiological, morphological, behavioural, life-history, even fitness itself. Personally I've used phenotypes from gene expression, which is one of the "lowest" (most close to genotype on the bridge between genotype and phenotype) phenotypic levels, through to lifespan, rates of ageing, and lifetime offspring production. Phenotypes are determined by genotype and/or environment (all non-genetic effects). For example, height may be affected by several genes, but it is also affected by nutrition during development and growth (among other things). As an evolutionary biologist, I feel the need to point out that selection operates on the phenotype, it is because genotype often has some effect on the phenotype that we see evolution by inheritance of genetic information.

Correctness of your statement:


  "Phenotype is the enzyme composition within the cell"


Would be sort of incorrect. It certainly wouldn't be appropriately defining phenotype as a general thing. It would be saying that phenotype means enzyme composition within cells, and the appropriate sentence depends on what you are trying to say. The following would say that, in the discussed case, the enzyme composition of the cell is what you consider to be the phenotype:


  "In this study the enzyme composition within the cell is the phenotype."


Whereas, the following defined the concept of phenotype:


  "Phenotypes are characteristics which could be, in principle, measured, such as the enzyme composition within a cell."


The problem of your statement is it makes it sound like phenotype, as a general thing, is enzyme composition within cells. It's kind of a problem of while enzyme composition of a cell can be a phenotype, not all phenotypes are enzyme compositions of cells. Similarly to all humans are mammals but not all mammals are humans.